Unfortunatlly, I am not that farmiliar with intels (been running amd for a long time). I can help you with some info though and perhaps someone with more knoledge of intels can fill in the gap. From what I can tell from a quick search, pentium 3 cpu's use socket 370, which only suppports sdram. Lets wait for someone more farmiliar with intel to verify that though. Along the same lines of a new motherboard, consider this.
Amd sempron 2400 (socket a) 72$ (ripoff)
socket a motherboard
cheapest on newegg 34.79$
a good brand motherboard 59.99$
1 gig ram (2x512), it wasn't worth listing the cheapest as it was not that much cheaper
good corsiar ram 69.99 http://www.newegg.com/Product/Product.asp?Item=N82E16820145440
Total is 180-210$ (roughlly)
You get a newer, beter quality motherboard, a much faster processor, and 1 gig of much faster (corsair pc3200 valueselect) ram.
I just quicklly picked stuff off newegg for example price, so do not take those as recomended parts. I just listed them to give you a ball park figure of price. It might very well be worth spending a few more dollars to go socket 754 or 939.
There are two kinds of motherboards that most systems use. They are atx and microatx. The only thing I found on your system so fat was uatx? You need to check into that (more info on your exact model of computer might help me find it). The above motherboards are atx. I also don't know if you have a video card or onboard video. newegg lists 12 microatx motherboards with onboard video (no vid card required) for between 35-60$ for socket a so that doesn't change price much. Fyi the onbard video is probably much beter than what is on your pentium 3 so you get an upgrade there too.
First thing, find if your motherboard is microatx, atx or other.
Second, find out if your power supply is at or atx (if it is at, it will have 2 6 wire conectors that go to the motherboard, if it is atx, it will have on conector with 20 wires).
My personal opinion, you are seriouslly throwing your money away if you pay mega inflated prices for obsolete equipment like sdram (unless you can find some used cheap). There are factors to consider though. Is your power supply atx, and if so, is it powerfull enough to handle the new equipment? Will my copy of windows xp work on the new motherboard or is it locked to the old computers motherboard?
I started with a piece of crap celeron computer 5-6 years ago, upgraded piece by piece (couldn't afford a good computer all at once), and I think it is the way to go (unless you can afford it all at once). You could actually build a system using some of your exsisting parts pretty cheap (win xp and a monitor are two of the bigest costs, but you have a monitor and might have windows xp). For that matter there are bare bones systems quite cheap too (doesn't include several things you already have so don't need). Custom gets you much beter equipment than prebuilt systems though.
If it is workable (have to check out you equipment more), to get a motherboard, cpu and memory, for the same price of 1gig of sdram, you have made major upgrades. for a little more money (socket 754 or 939), your upgrade can become even more substantial. Also, these new parts continue to be of use if you further upgrade your computer.
Any thoughts on that? Also, please tel us as muchs as possible about your current equipment (be detailed). Perhaps we can try to figeure how much of it is usable for now.