Ugh. Well, your options are pretty limited because 2006-7 Mac Pros use older EFI-32 ROMs.
The only "official" consumer level cards that work with your system are:
1. GeForce 8800 GT (2006-7/EFI-32 version)
2. Radeon HD 3870
3. Radeon HD 4870 (it works with any Mac Pro, despite what Apple says)
And that's about it. 😛
You can still buy the Radeon HD 4870 from Apple (last I checked, they're still shipping), but it's $350. The other two cards you'd have to find used.
Alternatively, you could try finding a suitable PC 8800 GT or 4870 and flashing it (and by suitable, I mean a card that's based on a reference design and has a proper ROM chip). There's several threads here that discuss doing just that.