How much gaming improvement will a Graphics card make.

I bought a Dell and stupidly didn't think about upgrading although to be fair I did check and it had a PCIe slot, but it was only a x1 (useless really).

The PC is 2.8 Ghz, 1GB RAM, and onboard Intel graphics. It runs Tombraider Legend ok & Call of duty, but will not run Call of duty 2, which is fair enough. Call of duty 2 appears to need graphics stuff that the Intel cannot supply.

So I'm thinking of getting a PCI nvidea 256Mb X5200 (i think thats right).

What I'm asking is how much improvement in gamplay will that give. Obviously COD2 will now work, but for the others will a PCI graphics card improve a fairly good spec PC.