Graphics card advice: Optiplex 745 DT

I am upgrading an Optiplex 745 DT.

It currently has E6600 CPU, XP Pro, 320 GB HD, 4 GB RAM, X1300 vid card, and the combo riser.

I want to:

Double the RAM, install Win 7 64-bit, and triple the graphics power.

1) What is the strongest graphics card (confirmed) that will run in this machine? (using its 280 watt PSU)

2) Why should I be limited to a low-profile card considering I have the PCIe x16 riser? The current X1300 is installed with the full-height mounting bracket and there certainly appears to be more than enough room for a full-height card.

Thanks.