Unless you truly need more memory (opposed to mere 'upgraditis'), spending a bit more for a brand new modern system will give you far better value.
I used a similar system like yours (X2 4600+, 2x1GB HyperX DDR400, HD3850 AGP) up to last year. It came to a point where I realised any little (but costly) upgrades I did would not bring the system performance up to par with today's systems (and demands). Case in point, I built my parents a new system (PhII 550 w/ unlocked 3rd core, 2x2GB DDR3, Win7 x64, etc) for very little money and it easily trounced the A64 system I spent so much money on over the last few years. Quite bitter pill to swallow but there you go.
Regards.