You would probably be better off with a single GF7800GTX 512 if you've got that kind of money to spend.
2 or more 4200 (or faster) AMD processors (obviously 64-bit) (as fast as physically possible and as many processors as physically possible)
You do realize that the 4200 is dual core, right? If you really do want mulitple dual core, you're going to have to go for a Socket 940 Opteron setup, which means SLI is out of the question, and you'll have to get registered RAM... not to mention the cost will shoot through the roof.
There's definetly no need for more than two cores though. The VAST majority of games are not the slightest bit SMP aware. Sure, you get a slight performance boost since Windows automatically balances threads between the cores (or CPUs), but we're talking about a minimal increase. As a matter of fact, I was just doing some benchmarking for a review a few days ago with BF2 on single core and dual core, by disabling one of my cores. The peformance gain with the 2nd core was exactly 3%, or 2 fps. Where dual core really shines is in non-gaming uses, but there's no need for more than dual-core, especially considering how it vastly bloats the cost.
And, if you're looking for "fastest," you want the 4800, not the 4200.
4 GB RAM to dual channel (whatever fastest speed is right now)
That's absolutely pointless if your main use is gaming. I will tell you that I've never seen a scenario where even 75% of my 2GB RAM was in use, and I run the most intensive games out there (like FEAR) at the highest settings. Not to mention that you'll be forced to use 4x 1GB DIMMs, which will drop you to a 2T command rate.
There are a handful of (non-gaming) applications where > 2GB memory would be useful, and if you'll be doing one of those you should know it.
Asus A8N32-SLI Deluxe, if you can get your hands on one.
500 GB SATA3 (3 is the fastest to-date correct?)
There's little practical difference between SATA150 and SATAII, but you'll probably end up getting it with a drive of that size.
Since you're willing to spend so much money, you might also consider doing a RAID solution. 3 250GB drives in a RAID5, or etc, to give some redundancy to your data.
Amazing power supply for all my crap
Seasonic S12-600W. Hands down.