Personally, I'd just recycle the (FUGLY*) plastic "tower" enclosure, storing the MoBo & goodies inside for a future hack! }. . . since you've got the 300 Mhz board in the 5400, there's nothing that the 6500 really has to offer that the 5400 doesn't, apart from a tower enclosure, one extra PCI slot, and the subwoofer. So yeah, keep the 5400.
. . . or, if you're not into hacking, at least store the spares in case your collectable Black Beauty (pushing it a little there too*) ever needs a brain transplant.
* hyperbole alert! [
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