Oh man.. sorry! I just totally delete my explination.. Give me a few more mins
Okay.. Anyways.. Shit.. I totally lost that.
Here it goes...
The 6800gs makes the whole machine more future proof. Sure with the 6600gt, that would suit you fine. Although, if you were ever to pick up another game, a bit more demanding then that it would be nice to have a better card.
Okay, with the workstations, you want at least 2gb ram, to turn off the page file. The page file runs off the hard drive. But with a workstartion, you are always using the hard drive. Saving.. opening ect.. And its really hard for your hard drive to do all that at once. But with the 2gb of ram, windows will turn off the page file. So your not using the VM on the hard drive; your using it straight from the memory controller. Hence, making loading, opeing, saving ect alot faster. And easier on the hard drive, so it has a longer life span.
As for the processor, if you are going to be using this for a multitasking machine. Get at least a 939 socket. And one of the X2 Processors. But as halo will be pretty much your only game, you could easily straight up get an Intel. As for that processor you have. Its still single core. Get a dual core. Its so much better. I can reccomend some to you if you like..