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.