Stick with 2gb or 4gb...but you wont regret going with 4gb....Vista is a hog, everyone I know that is running it is enjoying it alot more with 4gb....
If your really wanting the Crucial Ballistix then go for it, but you should keep it the same...a mixed voltage is no fun.
I just got some of this ram...In Canada and its still great Ram, great price too, yeah I know its a M.I.R. but Patriot has a decent rep for mailing checks back out
Two sets of that costing you 200.00 now...and a 80.00 check down the road is not to bad, and you have the performance of 4gbs.
Two sets of the Crucial Ram and your at 270.00 and thats after the M.I.R. rebate
Its really your call at this point, but my suggestion is to get 3gbs out of your mind and focus on 2 or 4. How much you spend and what you get is really your preference...