Your celeron is going to SEVERELY limit play at lower resolution when most things are still CPU bound. That said, your best bet is an inexpensive DX9 card like a plain 9600 (not the SE), the 9600 pro, or even a 9600XT if you want HL2 when it comes out.
Agreed, the 9600NP is prolly your best bang for buck. Any possible way to get a 1.4 P3? That would help you quite a bit and you wouldn't have to buy a new MOBO/RAM
Some games/game engines are heavily bandwidth laden, and you can't escape the fact that physics and AI are still fully dependent on the CPU, so there is no escaping that with any video card. If it were me, I would say a GF4 Ti4200 w/128MB RAM unless you plan on upgrading the rest of your system in the near future.