edit: word of advice, ditch the warez site reference in your post before having an admin remove it or worse ban you...
Vista, would be best, most XP games should work in Vista fine, the decision would come down to 32-bit versus 64-bit, one of the main differences between the 2 is that your not limited to the 3-4GB limit for system memory (RAM, includes your Video memory), of 32-bit if you go with 64-bit.
some games made that ran on Win98/2K & XP might not run right on Vista, but other than that, most should.
as to "Downloading" an OS, I wouldn't even mess with that, buy your copy would be the best thing, less likely something to go wrong with it, as well as getting into trouble, also you'd have a physical disk with the OS installation on it for re-installing.
pricing a system right now, its a SFF PC (small form factor, got most of the parts, but still need to tweak it a bit, adding in a monitor & other stuff with that 800-900 budget, won't get you a whole lot, maybe a lower mid range type system, but nothing really blazing...)
CPU:
http://www.newegg.com/product/produc...82E16819103046
Mobo:
http://www.newegg.com/product/produc...82E16813127027
RAM:
http://www.newegg.com/product/produc...82E16820231098
GPU:
http://www.newegg.com/product/produc...82E16814127284
Sound:
http://www.newegg.com/product/produc...82E16829102007
Hard Drive:
http://www.newegg.com/product/produc...82E16822144415
Optical Drive:
http://www.newegg.com/product/produc...82E16827136120
Case:
http://www.newegg.com/product/produc...82E16811133035
Power Supply: (went with this as it was the cheapest 24-pin PSU with modular cabling for the SFF case, on there)
http://www.newegg.com/product/produc...82E16817153048
Operating System:
http://www.newegg.com/product/produc...82E16832116204
Total comes to $883.90, can easily shave some pricing down,
slightly lesser video card
http://www.newegg.com/Product/Produc...82E16814127286
lesser case, same brand & quality, just less flashy & handle & airflow options, but otherwise same exact case...
http://www.newegg.com/Product/Produc...82E16811133045
those two options there drops it down exactly $100 less for the total coming to $783.90
leaving you with about $100 or so for a monitor, mouse & keyboard, as well as Wireless card... not much cash for all that extra stuff...
could lose the sound card & use the onboard crap, would save you $65...
drop the memory down to DDR2 667,
http://www.newegg.com/Product/Produc...82E16820609219
would save $15... not a whole lot.
could go with a DirectX 9 card instead of DX10,
http://www.newegg.com/Product/Produc...82E16814130062
save another $25 there...
HDD pricing is about as low as its gonna get, price per GB pricing pretty much ends about there, you can get less HDD storage, but the price difference is almost nothing...
Ditch the OS & save $114, but your not gonna get any help from most here on "Downloading" an OS...
could resort to using some crap basic case design, for the $20 or so range, as well as a lower priced PSU.
do you have a PC right now?
if so, whats the specs & everything, see if there is anything you can use off it on the new one, like the monitor & keyboard/mouse, etc etc... is there any parts in the PC that can be swapped to a new one...
that right there could save a bundle on a new system, its what a lot of us do.