The point be the ram runs at best at the slowest speed of any stick in the system---so you gain nothing by buying faster and more expensive ram. And if you don't believe me, go to the overclocking forums and ask---but in general rule one is never mix ram.
Playing with the bios is more black art rather than cut and dried science---and mixing ram just increases the degree of difficulty.
And given that the OP already has two gigs, going to 4 gigs memory will probably force a significant downclock of the ram on many mobo's. In certain gaming apps where ram speed is
critical, that downclock could prove fatal in gameplay. And there is also like a 3.25 gig memory limit with a 32 bit OS. So my advice is the OP would be---do mucho research to find out if such a ram increase will even benefit you.