Well is there a budget? that video card will not be able to play all the new games coming out on high. If you need some thing that will max out most games to come get the 5770, or the 4870, 4870 performs slightly better but no DirectX 11 or eyefinity. The 4890 is also another options. And the 5850 will max anything out today and you will be safe for 2-4 years.
For the OS, I would suggest Win7 64-bit, but if you already have a copy of XP Pro you can use, that's fine too. Just remember that a 32-bit OS will only be able to take advantage of ~3.25GB of RAM.
The differences between those 5770s are the cooler and brand.
From what I've heard, the non-reference coolers work better (reference ones are the boxy ones), especially if you have good airflow in your case. The Sapphire Vapor-X at the bottom of that page you linked probably runs the coolest; their Vapor-X series is known for allowing high OCs. If overclocking isn't going to be considered, any of them are fine.
Brand is important because of manufacturer warranties and quality; XFX are known for their double lifetime warranty. They, Sapphire, and EVGA are probably the only brands I'd pay more for (of course, EVGA being Nvidia-only, I wouldn't pay them at all ATM ).
BUT. At first I thought you were going for the 4850 due to budget. If you have more than ~$100 to work with, there are a few more options: the 4860, and the 512MB and 1GB versions of the 5770. The 4860 is a great card if you plan on OCing and don't care for the new series. If you'd rather have a 5xxx card, there's the still a two options for your res: the 512MB 5770 at $145 and 1GB 5770 at $160 (leaving out 5750s, because they may struggle). If you can afford a 1GB 5770, get a non-reference model. Asus, MSI, and Sapphire have a few.