More RAM the better in most situations.
Gaming, sound compression, video rendering and CD/DVD burning will improve greatly. Just using your computer to write documents, view e-mails, instant message or even listeming to MP3s...not much of a help to these types of applications.
The two best things you can do to a computer to improve performance without changing out the motherboard or CPU is get lots of RAM and a great video card.
To get ultimate performance (in gaming) do the following:
1. Upgrade video card
2. Increase RAM
3. Upgrade CPU
4. Finally, motherboard.
It's amazing what a new video card can do for an older system, or how a crappy video card in a kick ass system holds it back.
Soyo Dragon Ultra Platinum Edition powered by a P4 3.06GHz w/HT @ 533FSB
ATI Radeon X800Pro 256DDR <---Yeah!!! I finally got it!!!
1024MB PC3200, Creative Audigy LS, 2 x 40GB = 80GB RAID 0
Windows XP Pro, DOOM 3, CS, UT2004, BF1942 RtoR, CNC Generals
Finished Half-Life 2!!!!!!!!!!