There are several good places. One I and others have used is newegg, but they're not 100% the best. They're good, though.
First things first, you have to find out what kind of ram you have. If your mobo is that old your ram might be obsolete. Pull a stick out (discharge and static buildup by touching the frame of the case first) and look at it.
This is DDR:
http://www.virtualvisiontech.com/specs/computers/comimages/RAM.gifIt has numbers on each side, from 1 to 92 and 93 to 180-ish
This is PC133:
http://www.okcomputers.cz/foto/F3/MEM294.jpgIt has numbers only to 164.
If you have 164-pin, 2-notches, your RAM won't work in any new motherboards. If you have 180-ish (I can't remember the exact number)-pin ram, and just 1 notch, it is DDR.
Depending on the age of the RAM, even if you have DDR you might want/need to upgrade (PC 2500 will run very slow compared to PC 3200 and above). Second note: Some AMD motherboards take DDR2, totally incompatible with DDR, so if you get a motherboard with that you have to upgrade anyway. Bet on needing to buy at least a single stick of 512MB in your budget.
Your PSU will probably get you by, but you might need a new one depending on requirements for the motherboard/chip. Example: The Pentium4 line requires a 4-pin square connector to plug into the motherboard separately from the 20/24 pin or else your computer won't boot up. Things like that.
First decide what chip you want. Base it on preference, price, speed, whatever. THEN pick the motherboard with the USB, PCI and other connectors. It doesn't do much good to select the motherboard features then find it's got an obscure RAM type and/or only can mount older chips (see what I mean?). You mention a 256MB ATI, I'm assuming it's AGP based on the age of your motherboard. Keep that in mind -- if you want to use that card you have to get an AGP motherboard.
If you want front-panel connectors, you have to get a case that supports this. This comes with the case, not the motherboard. Most often the motherboard will have internal connectors to attach to the front panel. Just a heads-up. Case-wise, you also want a fan mount on the side depending on the processor (they need the extra air inlet directly over the CPU on P4's, and possibly AMDs -- I'm unfamiliar with the latter).
So, like I had to when I upgraded my 5 year old comp, you might need to get more than you planned on. Your components are good, but to get a better CPU you need a better motherboard. A better motherboard means our older RAM is no longer good. A better motherboard/CPU combo might require a better PSU.
So you need mobo/CPU
You need to assume you MIGHT need RAM/PSU as well. Need to at least consider that.
If I typed anything that was too obvious, I apologize. I wasn't sure how tech savvy you are (I'm not too savvy myself!) and so I erred on the side of caution.