People are saying Vista was pushed back to spring of '07.
I'm no fan of the "waiting game" so I'm going to buy now (within the month) and be thoroughly pleased with the purchase.
If you need to buy now, then go for it. It's not like if you purchase a Core 2 Duo now, it'll somehow be slower when Vista arrives. Quad-core solutions for CPUs won't be realized in the gamer-market for quite a time. If you're a power user who does video encoding, image editing, movie production and other demanding tasks (all at once!) ... the benefits of multiple cores will be more readily seen.
The technology array right now isn't revolutionary. Sure, you can spend an extra 400-600 bucks and shave 1 minute off when saving a movie file. Or you can say your computer saved you some odd 30 seconds of your life during a DVD compression task. But honestly, if you're more of a game enthusiast, the present-gen tech provides for more than enough power when it comes to handling today's (and tomorrow's) game applications.
If you're like me, and you stick to one or two games for a long time, then there's really no sense in waiting if you wish to buy now. True, you will save money by waiting, but that is an unending trend. Time depreciates value as innovations occur.
My rig is a generation old (see sig). I got it when DDR2 was first emerging in laptops, and 667 was ne'er to be seen. It was before the Core Solo's ... it's a Pentium-M "Centrino."
I'm mostly into the HL2 games and this computer is struggling to keep up at times. I'm upgrading to an E6600 with either an X1900XTX or an X1950XTX. Coupled with 2GB of low-latency PC6400 RAM and a high-end motherboard, my forthcoming machine will last me well into the future. The price should fall around $2,000 ... and that's quite attractive considering its potential longevity.
Go for the upgrade if you're really set on it. You won't be disappointed. But if you can wait 6-8+ months, then that isn't a bad idea either.