If the requirements posted on the nfs:the run website are to be trusted then you might be right, the requirements for the game indicate that windows vista or 7 is needed the only reason I can think of to do that is the lack of support for DX11 on winXP, but the game has not been released yet so it's hard to know for sure.
http://www.needforspeed.com/therun/faq#
The minimum requirements are as follows:
MINIMUM PC REQUIREMENTS:
OS: Windows Vista SP2 32-bit
DirectX: DirectX 10
Processor: 3.0 GHz Intel Core 2 Duo or AMD Equivalent
Memory: 3 GB
Hard Drive: 18 GB
Graphics card (AMD): 512 MB RAM ATI Radeon 4870 or higher performance
Graphics card (NVIDIA): 512 MB RAM NVIDIA GeForce 8800 GT or higher performance
Sound card: DirectX Compatible
Keyboard or Gamepad or Steering Wheel
DVD Rom Drive
Online: 512 KBPS or faster Internet connection
RECOMMENDED:
OS: Windows 7 SP1 64-bit
DirectX: DirectX 11
Processor: 3.0 Mhz Intel Core 2 Quad or AMD Equivalent
Memory: 4 GB
Hard Drive: 18 GB
Graphics card (AMD): 1024 MB RAM ATI Radeon 6950
Graphics card (NVIDIA): 1024 MB RAM NVIDIA GeForce GTX560
Sound Card: DirectX Compatible
Keyboard or Gamepad or Steering Wheel
DVD Rom Drive
Online: 512 KBPS or faster Internet connection
From that chances are you'll need to upgrade to Windows 7. but I'd wait until the game is released to be sure so you don't upgrade for no reason.
As far as your original question is concerned, I wouldn't go above 8Gb or ram right now there's usually no need for more in most scenarios, and most of the time 4gb is sufficient, although I still recommend going for 8 Gb when budget allows for it, mostly for future proofing and because of the fact that ram is so cheap.
Keep in mind that 4Gb would be sufficient for NFS : The run if that's all you're interested in