If you're willing to spend the money, Core 2 Duo is the best choice at the moment. I personally recommend the E6600 2.4Ghz.
Buy 2 gigs of RAM. Buy it together if possible so you get a matched set. If not, you'll want to upgrade to 2 gigs ASAP. This is especially true if you intend to run Vista, as it's quite resource-intensive.
DDR2 800 RAM is obscenely expensive right now, due to all the people buying it for C2D and socket AM2. If you want to save some money and go with a Core 2 Duo, buy DDR2 667 RAM instead. There's almost no performance difference and you'd save a good chunk of cash if you buy the standard 2 gigs for gaming. If you buy a socket AM2 CPU, then you must buy DDR2 800 as you'll take a distinct performance hit with the 667.
Be wary of the DX10 cards. I've seen photos of a Nvidia prototype that required watercooling and had two connectors for the PSU. Expect both the ATI and the Nvidia cards to run extremely hot and power-hungry. That's an unfortunate trend, as both major CPU makers have realized low-power is the way to go. Basically, if you plan on using a DX10 card, you better have a pretty beefy power supply and good case cooling. Then there's the usual new-gen card issues, bugs, etc. I recommend buying a solid mid-range DX9 card (geforce 7600GT, 7900GS, or their ATI counterparts). Upgrade to a DX10 card once the initial issues are worked out. Now, if you *do* have a good PSU and good cooling and cash to spare, by all accounts both the Nvidia and the ATI next-gen cards are extremely fast.
The lower your budget, the more attractive AMD becomes as an option. Current games aren't really limited by the CPU. If your budget is under $1000, socket AM2 starts to become a really useful alternative. It *is* slower than C2D, but it's cheaper by enough of a margin that you can more than make up the difference by investing in a better videocard, more RAM, etc. If you want to stay under $850 or so, you might want to consider socket 939 as it performs 95-98% as well as socket AM2 and doesn't require expensive DDR2 800 RAM. Note that socket 939 won't be useful for future upgrades, so keep that in mind as well.
Do what your budget permits.
J_A_B
