I enjoy first person shooters and enjoy a good RPG. What are must get for PC?

I can play any game.

My computer came with no operating system and I am looking to buy one. I don't want to mistakenly buy an upgrade.

is this ok? http://www.amazon.com/Windows-Ultimate-64-bit-System-Builders/dp/B0015CCG4U/ref=pd_bbs_sr_2?ie=UTF8&s=software&qid=1214412354&sr=8-2

What operating sysytem would be most compatible for gaming with this pc?

http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=250256438691

Please look through and check if they are goo. :) Im not so computer smart.

Case: http://www.newegg.com/Product/Product.aspx?Item=N82E16811103010
XCLIO A380BK Fully Black High Gloss Finish SECC 1.0mm thickness ATX Full Tower Computer Case

HDD: http://www.newegg.com/Product/Product.aspx?Item=N82E16822148316
Seagate Barracuda 7200.11 ST3320613AS 320GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive

Video Card: http://www.newegg.com/Product/Product.aspx?Item=N82E16814150252
XFX PVT88PYDF4 GeForce 8800 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card

Power Supply: http://www.newegg.com/Product/Product.aspx?Item=N82E16817139001
CORSAIR CMPSU-520HX 520W ATX12V v2.2 and EPS12V 2.91 Power Supply 100

Operating System: http://www.newegg.com/Product/Product.aspx?Item=N82E16832116485
Microsoft Windows Vista Home Premium SP1 32-bit English

CD/DVD burner: http://www.newegg.com/Product/Product.aspx?Item=N82E16827136144

Mother Board: http://www.newegg.com/Product/Product.aspx?Item=N82E16813131232
ASUS P5N-D LGA 775 NVIDIA nForce 750i SLI ATX Intel Motherboard

CPU: http://www.newegg.com/Product/Product.aspx?Item=N82E16819115037
Intel Core 2 Duo E8400 Wolfdale 3.0GHz 6MB L2 Cache LGA 775 65W Dual-Core Processor

Heat Sink: http://www.newegg.com/Product/Product.aspx?Item=N82E16835186134
ARCTIC COOLING Freezer 7 Pro 92mm CPU Cooler

I still need to decide on the RAM memory.