My Best GPU Option?

I recently got another case and some supplies, so I am rebuilding the computer from the brink of death. Honestly, the Specs on it are not all too horrible, except that when I looked to begin searching for graphics cards, I came into some problems. My Motherboard only has an AGP x8 and a PCIe x1 slot. [It has other, normal PCI's too.] It is a Dell Optiplex 745 case, with a rather large Heat Sink and fan cooling a Pentium D 3.00 GHz Dual Core processor. The Motherboard is a Dell Foxconn LS-36. All of it's drives are SATA, the processor is nice, and it has 4 DIMM slots, so I really like EVERYTHING about it EXCEPT that is lacks horribly in compatibility for good GPU's.

I was hoping to find at least a decent GPU that would work in the slots I have, and allow me to play the games I normally enjoy now, like Minecraft, L4D2, TF2, and other games that aren't necessarily graphics demanding. Any help I could get is greatly appreciated. I know that I will more than likely not get something good, but I really do like the rest of the case. It's large, open, and has great airflow.

Additional Info: Windows 7 32-Bit, 305 Watt Max Output, VGA Output Preferred

Thanks!