I need to upgrade some parts of my computer but I do not know what to upgrade first.

AMD T-Bird 1.2 GHZ OCed to 1.44 GHZ
Epox EP-8KHA
512 Mb PC-2100 ram
Leadtek Geforce 2 GTS 32mb DDR
SoundBlaster Live! Value
Lite-On 48x CDRW
Maxtor 7200rpm 40GB

I have $400 to spend, what should I upgrade first? Thx.