Any chance my motherboard supports the HD 4850? And...

It will work fine in your motherboard its your PSU I would be worried about a 400 watt PSU should be enough power but if its a real cheap no name PSU it might have problems. ATI recommends 450 watt but it can be run on less as long as you have at least 28 to 20 Amps on the 12 volt rail it should be fine.

Can you list the make and model of your PSU if you can if you cant find that there should be a sticker on the side of the PSU that should tell you what it can put out.

Your CPU is a bit old but it should still perform OK it might be a little bit of a bottleneck but it should be fine if you can overclock it as much as you can.

I just had a look at the sticker and it show's "Power King Vista 400w". Short after, I searched it up on Newegg and this came up.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

I'm guessing it's a cheapo PSU.

Surprisingly I never encountered any problems with the PSU for 2+ years.

Well thats a problem that PSU will most likely fail if you stress it also it doesn't even have the required 6 pin PCIE connector for the card and I dont suggest using an adapter also cant even find any specs on it. You will have to upgrade.

Check out this PSU its about the cheapest power supply I can suggest anything less will be really bad.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Sounds good.

I'm surprised I never came across that PSU when I originally built this computer.

That Corsair puts out just as much power as some 500 watt units put out If you want a little more juice for further upgrades get the 550 watt Corsair.

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

This unit will power just about any Single card out except the 5970 and GTX 480. And if you want more info check out the link in my sig.

Yup, you're gonna have to upgrade that PSU. Even though you haven't had a problem yet, the Radeon 4850 consumes more power than a lowly 8600GT. As for the CPU and RAM, yes they would bottleneck your 4850. Well, you should be o.k with 2GB of RAM in most games assuming you have a lean install and don't have too many background services running. As for the CPU, it's a bit slow for a 4850 so you may want to consider checking up on the CPU support for your motherboard, or at the very least over clocking your Current CPU a bit.