2Gb is workable for RAM on a Win7 machine.
4Gb is advisable once you start to do other things, such as using large installed programs.
Anything above 4Gb is only ever going to be addressable if you have the 64bit version of Windows 7.
8Gb is probably overkill unless you're doing a lot of photoshop etc.
Don't completely trust the Crucial tool, research the motherboard's capabilities with the manufacturer. However, 4Gb would suit, but I would suggest a defrag of the hard drive and probably spending some of the cash saved from not going to 8Gb on an SSD would give the best bang for buck.