Ya I am kinda in the same boat. I have a laptop that plays my games ok, and a desktop that is suffering. Its an old hp Pavilion and is pretty much at the max right now. thats when I found this:
Newegg.com - VisionTek VTKX1300256PCI Radeon X1300 256MB 128-bit GDDR2 PCI Video Card - Retail
I think that this card is a great value, even though it is $100.
I am normally an nVidia fan but his is the best NewEgg offers for PCI.
If you could afford the extra $25 I think that this card would make you pretty happy.