I'm not sure what you are using to make it "bottleneck" but the celeron is a budget processor (lower end). If you are looking for some power I would look for something in the pentium or Core2Duo line (such as a 4300 or 4400) if you board supports it.
The main question to ask yourself is, what are you using the PC for? If its for games and you want more "umpf" to it, you'll need to upgrade the processor. If you are just doing your basic office and windows tasks, you should be fine with what you have.
I can't even find your motherboard on Biostar's site. The closest I see is P4M800-M7. Beyond that, I'm no expert on Intel processors from 2-3 generations ago. Just setting some info up for someone else that is better informed on this.
If you're on a low low low budget, then a 9500 series ATi card, preferrably Pro, off eBay is good enough. Just make sure you have a power connector that connects the PSU directly into the card, or if the card comes with one.