Does it matter what Windows version I get?

Currently, I've been using XP on my old junk computer for 7 years (PSU dying, etc).

I don't really want to pay $90 for Windows 7, as Windows XP is perfectly fine.

Does the OS effect performance? Does it change game speeds? What does it change other than layout and sounds? I'm a tad bit more hardware smart than software.

This is my build:

(2/12/13) Updated, $1320 (average ~$1250)

($224)CPU: Intel Core i5-3570K 3.4GHz Quad-Core
($30)CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing
($135)Motherboard: ASRock Z77 Extreme4 ATX LGA1155
($60)Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600
($440)Video Card: XFX Radeon HD 7970 GHz Edition 3GG
($27)Sound Card: Asus Xonar DG
($60)Case: Zalman Z11 Plus ATX Mid Tower
($9)Case Fan: Apevia CF14SL-BBL 59.6 CFM 140mm
($95)Power Supply: Rosewill 750W ATX12V / EPS12V
($22)Optical Drive: Lite-On iHAS124-04 DVD/CD Writer
($90) Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit)
($86)HDD: Samsung Desktop Class Spinpoint F3 1 TB SATA 3.0gb/s 32 MB Cache
($12)HDD Cooler: Startech Dual Fan HDD Cooler
($25)Fan Controller: NZXT Sentry Mesh Fan Controller w/ 5 Channel
($8) Paste: Arctic Silver 5 High Density Silver Thermal Paste