Okay - here's the deal - I was in exactly the same boat as you two months ago.
I'm assuming the hard drives and DVD drive and so on are all working - so you just need a motherboard, CPU, and of course, the graphic card. Oh - and get a copy of WinXP Pro if you don't have it - it's great.
Motherboard - needs to be an Intel chipset and an ASUS motherboard. This combo with an Intel CPU makes for a very stable and fast system.
CPU - The Celeron D 356 is a good choice. Overclocks to 5Ghz quite easily if you want, or you can use it as-is. This is a cheap stopgap measure, afterall. The idea is to get a good working system now and later upgrade to a better processor if you want(as well as better memory and so on). $80.
You will need an aftermarket cooler, though, if you plan to OC it, and better memory, so I'd leave it at 3.33Ghz for now - it'll run about like a 3.0Ghz P4 Northwood.(512K cache - totally new Celeron design)
When you get better memory in a few months, drop this cooler on it and boost the clock to 200Mhz.(just over 5Ghz) - it'll get a 3dMark score of 8-10K quite easily.
- can install a better dual-core CPU later on.
- can install a better CPU cooler and OC the chip you have.
- can handle very fast memory later on.
- has one IDE connector on it - but can handle SATA later on as well.
- Video card is already taken care of.(grin)
EDIT: This appears as if it won't use your current memory(saw 400, thogh DDR2/400 - my bad. I'd just get a gig of cheap DDR2/400 for now and upgrade later if that's the case.
1. i bought the exact same x850 xt not loud at all, i dont know why people complain. idles 40deg c at 5% fan speed, very quiet.
2. intels low end offerings suck ass, unless your going to oc a pentium d i suggest you stay away, a64 3000+ is a good choice. i mean cmon even the sempron pwns the celeron D. who wants a 100c deg idleing chip? with crap performance, at 5Ghz.
personally i love my x850 xt i just played throught and beat prey(insanely awesome game) 1280x1024 max details 2xaa 60fps the whole time, css 1280x1024 6xaa 16xaf, you cant go wrong with how cheap it is. Peace.