My understanding is that it's only the last RAM slot (away from the PCI slots) where this is an issue, so 2 slots will take higher DIMMs, and that removing* the fan guard makes the problem go away altogether. Hacker beware, this is only a vague memory.

* Snip it off with side cutters as close as possible to the outer edge, and fold any remaining metal flat out of the way. Grinding or hacksawing may lead to stray metal filings in the computer and/or power supply