Improve older single core laptop by o/s upgrade from winXP to win7

Not really. Windows 7 is a better optimized and faster remake of Vista, but it's still more demanding than XP. That laptop could probably run 7, though not particularly quickly. One other thing I'd check on is driver support, particularly for the video chip. If there's no "proper" video driver for Windows 7 on that older model it will function on the basic Microsoft driver built into the OS but graphics performance wouldn't be ideal.