I believe they make an FX 5700 LE PCI card...but as we all know, the nvidia FX cards blow, and anything with LE on it blows as well.
As for CPU, you have a 2.6ghz Celeron. Celerons are stripped down CPUs, not as good as the higher end Pentiums (im going 2 leave AMD out of this one). There are specific differences, but to keep it simple, celerons suck. Most people here have a very low opinion of celerons.
a 2ghz P4 would prolly be better than a 2.8gh celeron.
I suggest doing a complete rebuild. Start from the ground up