Should I upgrade the ram in my setup?

I have a Dell Inspiron 530 with a quad core and 2 gigs of ram @ 800 mhz - I am looking to upgrade to 4 gigs using a $100 giftcard I received from dell - I am aware of bus limitations on my board that could inhibit the ram from reaching 800 mhz (more like 667 mhz) - given this flaw in the dell mobo bios, is it worth it still to upgrade my setup to 4 gbs of ram? Which is better: 2 gigs @ 800 mhz or 4 gigs @ 667 mhz? Also, I am aware of the 32 vs 64 bit limitations wherein only 3.5 gigs will be recognized on my xp home os - however, if I were to upgrade to xp 64 pro, would this increase productivity as well? I am using my setup mainly for music production and watching movies/videos. Any suggestions on configuring the virtual memory to best suit my needs? Feel free to debunk any of the info I may have erroneously provided - The most complete answer will get the points. Thanks