If you really do want to go to the extreme on RAM, i7 is the way to go. It's the only chip you can hit 12gb RAM on without having to shell out for 4gb sticks. Also, to save a few bucks skip over Vista and install the Win7 64 bit RC. It's free (until march) and in general just kicks ass for an OS.
So I'd go with something like the following:
Intel i7-920 > $280
Asus X58 Mobo > $210 ($15 mail in rebate so $195 after)
OCZ 12gb RAM kit (6x 2gb) > $170 ($20 mail in rebate, so $150 after)
That gives you a total cost of $660, $625 after mail in rebates. (All items have free shipping)
If you'd be satisfied with 8gb RAM you could go a lot cheaper with an AMD Phenom 2 setup, but if you want 12gb you'll have to use 4gb sticks in any non-i7 setup, and that means you'd be spending at least $460 on RAM (much lower quality ram at that), leaving you with an extremely cheap CPU & Mobo to come in any less than the i7.
Here's a Quick comparison of what you could get away with spending if you limited yourself to 8gb RAM:
AMD Phenom 2 940 > $190
Asus AM2+ Mobo > $80 ($10 mail in rebate, but no free shipping)
Patriot 8gb RAM kit (4x 2gb) > $100 (No free shipping)
That gives you a total of roughly $370 for the AMD setup with 8gb RAM. Over $250 less if you can live with lowering yourself from 12gb RAM to 8gb RAM.
Which is the better way to go... it's hard to say really. I'm a huge fam of the i7 myself, it is certianly leaps and bounds better than any other processor on the market currently, but you pay for it. Both in the cost of the CPU, and of the motherboard. For that $250 difference you could replace your ageing 8800gt with a GTX275 or ATI HD4890, which would leave your system in a much better state for gaming in general.
But on the other hand going with th i7 now would leave you with a system that could very easily be turned into a top end rig with just a new video card at a later date should you so desire. I guess it ultimatly depends on your budget, and what you forsee in the next 6-12 months for any additional spending money.
As much as I'd love to convince you to spring for the i7 (cause it just rocks), I would have to probably advise going with the AMD setup (8gb RAM will be fine) unless you forsee getting the extra $ to properly outfit the i7 with a new video card by the end of the year. That prosessors greatness would be ultimatly severly bottlenecked by your current GPU.
Anyways, this is getting too long, but theres a couple different ways you can go at least.
