Should i buy win 7 or re-install win vista

Hello…
Please help me with this technical problem,
I owned a Dell Latitude 2.0 ghz, 3GB memory, Core duo, 300GB HD that turn out to be very slow and I am going to make fresh install of windows.
Should I buy Windows 7 or will this resource tough for this PC?
Or will I reinstall Vista that came with the PC?
Thanks for your response….
