So as I understand it you are wanting to know how the summer 2000 iMac 500 with 16Mb VRAM compares with the previous variant with 8Mb? 
I cant give an entirely educated answer one as I have only used the 500/8mb and 600/16mb boards... havent thus far come across one of the 500/16 variants as yet to make a comparison as they appear to be a fairly uncommon variant round these parts.
The combination of 100mhz gain in speed and better video with more VRAM tho I can say with certainty makes for a very noticably better experience, particularly running Tiger, as I found out by running the same hard drive and system before and after the 500 indigo was rebuilt into the 600 snow. I can't say for certain how much of the gain can be attributed to CPU speed vs improved graphics controller, but I can only imagine that the better graphics would go some way to improving the fluidity of the UI, particularly in Tiger which as a whole seems quite a bit more graphics intensive.
If you have the board available to you tho, by all means I'd recommend doing the upgrade, as there is little to lose, and it's a reasonably straightforward swap once you know what you are looking at. One thing I must stress however iis that you check the placement of the heatsink before reboarding any slotload iMac with a later or earlier revision board, as there were changes to the chassis midway through production to accomodate a revised CPU location on the logic board. As far as I can so far tell, the 350/400/450 models all shared the same heatsink and logic board configuration, with a raised sink moulded integrally into the chassis where the CPU sits with the board in situ. the 500/600 and presumabley 700 models however have a solid metal block on the chassis for a heatsink that is in an entirely different location corresponding with a revised logic board, meaning that if you interchange boards/chassis between early and late revision machines, even though the board will screw straight in, the CPU will burn up due to absolutely zero heat transferrence off the chip... SOmething people tend to forget about iMacs is that they are convection cooled, and the entire metal chassis forms a heatsink away from the CPU.
As I said, best I can tell their are only two revisions and they are clear cut with no cross over between processor speeds, but its best to check and make doubley sure, even dummy fit the board to check for contact, before powering up.
And yes the internal bezels become brittle due to the heat they are subject to and are prone to cracking particularly if the machines are dropped or treated roughly later in life. This graphite of mine is so badly cracked internally that I cant actually put it back together properly until I replace the inside bezel... as soon as i removed the case plastics the inside literally disintegrated all over the floor. I'm going to use the bezel off an early DV I have here that wont power up to replace it, as the early ones from me experience seem to be made from a different plastic compound that is far more durable than the later ones and doesnt become anywhere near as flakey and brittle.
I cant give an entirely educated answer one as I have only used the 500/8mb and 600/16mb boards... havent thus far come across one of the 500/16 variants as yet to make a comparison as they appear to be a fairly uncommon variant round these parts.
The combination of 100mhz gain in speed and better video with more VRAM tho I can say with certainty makes for a very noticably better experience, particularly running Tiger, as I found out by running the same hard drive and system before and after the 500 indigo was rebuilt into the 600 snow. I can't say for certain how much of the gain can be attributed to CPU speed vs improved graphics controller, but I can only imagine that the better graphics would go some way to improving the fluidity of the UI, particularly in Tiger which as a whole seems quite a bit more graphics intensive. If you have the board available to you tho, by all means I'd recommend doing the upgrade, as there is little to lose, and it's a reasonably straightforward swap once you know what you are looking at. One thing I must stress however iis that you check the placement of the heatsink before reboarding any slotload iMac with a later or earlier revision board, as there were changes to the chassis midway through production to accomodate a revised CPU location on the logic board. As far as I can so far tell, the 350/400/450 models all shared the same heatsink and logic board configuration, with a raised sink moulded integrally into the chassis where the CPU sits with the board in situ. the 500/600 and presumabley 700 models however have a solid metal block on the chassis for a heatsink that is in an entirely different location corresponding with a revised logic board, meaning that if you interchange boards/chassis between early and late revision machines, even though the board will screw straight in, the CPU will burn up due to absolutely zero heat transferrence off the chip... SOmething people tend to forget about iMacs is that they are convection cooled, and the entire metal chassis forms a heatsink away from the CPU.
As I said, best I can tell their are only two revisions and they are clear cut with no cross over between processor speeds, but its best to check and make doubley sure, even dummy fit the board to check for contact, before powering up.
And yes the internal bezels become brittle due to the heat they are subject to and are prone to cracking particularly if the machines are dropped or treated roughly later in life. This graphite of mine is so badly cracked internally that I cant actually put it back together properly until I replace the inside bezel... as soon as i removed the case plastics the inside literally disintegrated all over the floor. I'm going to use the bezel off an early DV I have here that wont power up to replace it, as the early ones from me experience seem to be made from a different plastic compound that is far more durable than the later ones and doesnt become anywhere near as flakey and brittle.