The Rig I recommend will be shooted by each one over here. But at the end of the day you will realize that you will get the maximum performance in the current crop of games and hopefully do better even in the future.
AMD 64 3500 Venice S939 : should be around 5k, or maybe 5.5k
A dual core is not in your budget, for that you will need to cut down on the graphic card, which is not going to help.
You can overclock the VEnice around 2.4-2.8GHz and then it is more or less good enough for most games out there. Even at stock, it won't be that bad, but overclocking will help. Get a cooler master or antec or better cooling, around 1-2k if you want to get the maximum out of your system. Else stick to stock cooling and you will hopefully manage 2.4-2.5GHz or more.
Asus A8N E, should be around 5.5k
VFM, will help you with overclocking, stable
2 X 1GB DDR 400 Transcend(UCCC chips-really overclockable), around 9-10k
I know DDR is dying, but in the next upgrade you can do the entire thing, at the moment, this will do. Are you upgrading before 1.5-2yrs BTW?
Geforce 7900GT, must be 17k+, or maybe around 18-20k
Seagate SATA II NCQ 250GB, 3800rs
Sony DVD RW
one combo as well, choose the brand, not sure
Sony FDD
total: around 4k or so
Cooler Master 450 watts PSU + a local cabinet, all this makes around 4000+
total so far is, >50k, <55k
If you want you can add:
Creative sound card, will tell you if you want, around 1.5-2k or so
Altec Lansing 251 (5.1) speakers, around 4k
Microtek 800va: around 2k
If you want dual core, add around 3k to get X2 3800, I don't know how much you need to add for X2 3800 AM2 motherboard because I don't know the price of oc mobos for it. The problem is that for about this price you do get the CPU+MB for Intel C2D, but you need to spend more for RAM(maybe not actually!), and you don't know if the motherboard is overclockable or not.
If you can stretch your budget to 55-60k, then you can get 2 X 1GB DDR 2 Transcend/Corsair Value Select(preferable)(using d9 chips). former should be around 10k
Intel C2D E6300, around 10k, someody is selling almost new for sub 10k
a mobo which overclocks to atleast 2.1-2.2, not the asrock, though it might oc, but doesn't have dual channel and sata II, forget that, if oyu find that kind of oc mobo which is stable after oc as well, you can consider that. and upgrade mobo and all later. that is the best.
but don't compromise on the 7900GT, let that be a constant