The first thing you need to do is chuck that Celeron and get a Core 2 Duo. I checked your motherboard's compatibility list, and if you can find one still, a Core 2 Duo E6700 is relatively inexpensive at the moment (anywhere from $80-$120, being that it's an older Core 2 chip, that's actually not too bad)
Unfortunately there's a snag on this next bit. Your RAM, according to the motherboard maker, is maxed at 2GB. You could buy more and see if the system will run with it (2 x 2GB modules) but obviously no promises.
Which Sapphire 4800 series card is it? 4800 is too vague - that's just a family (generational) number. 4850? 4870? etc. Need to know more. If you don't know how to check, just download this program: Speccy - System Information - Free Download
and post a screenshot of the first window when you run the program.
Also, which operating system are you currently running?
How much are you hoping to spend?