The obvious "option" is to go with a new C2D system, however there's nothing in the way off SFF overclockability for C2D, so the gap isn't going to be as big as it would be with a desktop.
Luckily it also appears to have a PCIe slot, so it might well be worth going for an X1950Pro or X1950XT on the excellent this week only offer. Past that you're going into 8800GTS 640MB territory, and you may find your CPU a bottleneck there.
I'd also recommend picking up another GB of RAM, maybe swap your HDDs for one big one, a Seagate 7200.10 possibly?