The 7300 was old back in 2006 already. It is for basic office stuff and 2D. It had 256MB video memory, not the 1GB of the 5770, or the 2GB of many now (Apple had trouble with anything more than 1.5GB for years).
I can't tell you what is best but for what you listed, check out MacVidCards, he has 6870 for Mac $225.
http://www.ebay.com/itm/ATI-Radeon-6870-1-GB-for-Apple-Mac-Pro-/261288591074
http://www.ebay.com/itm/261264795049
http://www.ebay.com/usr/macvidcards
Some people keep the 7300 in another PCIe slot to use if needed to see what is happening early on but use a higher end card for use.
Only limit is try to find a card that does not need 8-pin aux power. You will have to buy one or two of the 6-pin cables to power a card which are only $12-15 each.
The only difference in 2007 2,1 was firmware support for 8-core 53xx processor. Identical in memory, graphics and everything else, though it req'd 10.4.9+ special build on OEM DVD (10.4.10 or above recommended).
If you don't have a pair of 5355's now you should, 8-core 2,.66GHz and then run better.
But building a PC with Core i7 4770 with mobo and the rest would be more useful and run better. Of course the brand new "Late 2013" Mac Pro "nMP" $4K would run Windows (8.1+ only which is what I use) just wonderfully and has the power you need for video and graphic editing, and 6 or 8-cores as well as 4-core
I really wonder if you want to keep the 2007. Okay. There are $25 2 x 2GB FBDIMMs now that are much better. $100 for 16GB. You need that. You should be running 64-bit of course and that was not easy or supported in these. Thermal fan control under Windows is pretty much totally lacking and a real concern of mine and using it under stress. One reason I left using it for Windows behind.
GPU and some RAM. And put Windows or any OS on a nice fast SSD makes a difference. But enabling AHCI should be done and I am not versed in how best to accomplish that. It does matter, especially with SSDs but for any hard drives.
Paragon Software has some good programs to manage and clone Windows to another drive if you need to or want a better system restore image and cloning software.