Ram support over 4 GB (over 3 GB to be practical about it) is one of the few things Vista would be worth upgrading for, if you need that feature. The previous poster is right, for how cheap ram is right now, maxing out your MB's capacity isn't crazy expensive like it used to be. Most modern MBs support 8GB.
Play any multi-player games where the first to load a level has the advantage? Create a ram drive, install the game to it, back up the files to hard drive. Every time you play, copy the files back to the ram drive from your hard drive (presuming you've rebooted and lost the ram drive contents since last time). You'll load levels, textures, etc so fast the I/O won't be the bottleneck any more. Pretty friggen sweet.
I'm sure there's tons of apps, that even if they won't access more than 4 GB of ram (or more likely if they're written for XP, 2 GB) that would run a lot faster with a ram drive. Its an old concept that people have mostly forgotten about 'cause it hasn't really been needed or affordable for a while.