One precaution. Do not (solely) rely on the Windows 7 experience rating for gaming (control panel/performance information and tools). I bought a fairly high-end laptop with a Windows gaming experience of 6.1 with the thought of running Trainz on it instead of a fairly low-end HP desktop that I had with a Window gaming experience rating of 3.0. Neither have a dedicated graphics card. Both have 4GB RAM. Desktop running Vista 64-bit, laptop running Windows 7 64-bit.
I put both PCs side by side and ran the same Trainz route on both, and amazingly the low-end HP had less stuttering than the high-end laptop, even though the laptop had a much higher gaming rating. Maybe it was hard drive speed - I can't explain it.
My next PC will have a dedicated graphics card for sure.