The system board should support more than 4GB. See  en.community.dell.com/.../19249074.aspx I don't know if some of the earlier versions used anything other than the G33 mother board, but if so, it probably will not support >4GB. Make sure than the added RAM is identical to what was in there initially: same brand and timings.
First post here, so hello everyone! Haven't had to post any problems in 4 years about my dell! Ok so here is the problem.I'm running 64 bit Windows 7 (fully up to date), I have the most recent bios update (can confirm it shows the correct version on bootup), and Win7 is still showing 3.25GB usable. I can confirm that Task manager also reports this number.I have 6GB installed in my PC, 2 x 2GB on one channel, 2 x 1GB on another.    Updates to this spec are: Vista replaced with Windows 78600GTS replaced with RADEON HD 5570 CuCoreRAM upgraded to 6GBSecondary hard disk upgraded to 1TBPower supply upgraded to 600WWiFi card removed, now using D-Link Wireless N USB dongleDelta66 soundcard installed with Omni I/O break out boxMany thanks for your time and help, best regards,Ash. 
3.25GB Memory cap in Win7 x64 (even with new bios) - Dell Vostro400