Are you running 2x1GB and 2x512MB for your RAM - you'll lose dual-channel operation if you're not.
Not so much of an issue, but you'll get a bit of a performance boost if you stick with dual-channel (i.e. matched pairs of DIMMs).
W7 will run fine though on 3GB, but it could be worth tweaking your RAM while DDR2 is still available and not too expensive.