Planning to add to your current RAM? If you really need more than 2GB right now I would buy 2 512MB sticks (assuming you are using 2 of 4 slots now which is a big assumption) to bump you up to 3GB. You'd be wasting some of the RAM if you went to 4GB, and I've ran Vista HP 32bit on 3 gigs (2 1 gig sticks and 2 512 sticks) for a long time and it was fine. I'm now running Win 7 64bit on that same setup, again with no issues, both performed well.
It is generally not advisable to run 2 different brands or timings or speeds of RAM, but in many cases it works without issue (my setup above is an example). I recommended the 2 sticks of 512 because of the dual channel thing, you don't want to run an odd number of sticks. The price of DDR2 is up everywhere too (like you said) so unless you are running several RAM hungry programs at once I don't think it is wise to put 4GB in there. Your next system probably won't even run DDR2 so its not like you can reuse the RAM in a future build.