I'd say 2Gigs is enough for most normal-consumer uses like games/videos/etc. Next summer probably 3 gigs. The summer after that probably 4 gigs.
The problem for me is OS. Buying an OS during a transitional period sucks. The old one becomes obsolete and phased out with new features and things that only work on the new OS. But the new OS is buggy, has driver issues, and has serious compatibility issues with many games and peripherals and programs. It's like being between a rock and a hard place - either way you lose and don't get all the features either due to bugginess/compatibility issues, or because of being phased out and obsoletion. I'd say if you have XP, keep it till at least SP1 for Vista comes out. If you have 2 gigs, I'd say keep that. Wait for Penryn/Phenom and SP1 and Geforce 9xxx. Of course in PC business something new is always around the corner, but the reason I say the above is because current mainstream is barely able to run Vista, low-end can't even run it, so you need a upper-mainstream or high-end just to fully use the operating system, not to mention play any games on top of it. Early next year seems like the best time to buy, when mainstream systems will start catching up to the OS requirements, and when PC's are not struggling as much with the bloatware.
I guess the good thing about bloatware is it forces people to upgrade PC's. This makes money for PC manufacturers, which helps the PC industry to research and hurry up and advance. If it wasn't for games pushing the requirements all the time and bloatware Windows doing the same, I doubt there would be nearly as much progress in PC industry, it would be crawling.