I'm in the same boat in trying to upgrade my brother's computer. The first thing you want to do is check the memory via CPUz (free download). It showed his comp of only running at 991mhz. When I removed the 2 sticks of PC4200 and replaced them with my OCZ PC9200 Reaper, it jumped to 2.2ghz which blew me away. Must be an AMD thing with the Ram vs. processor speed. If you have the same results, toss that old ram for some good low latency DDR2-667 or above (especially above if planning to upgrade the motherboard).
You seem to be headed in the right direction. After the ram, get a good power supply to power the graphics card. The Galaxy card you have should work.
If you have the slower 5400rpm hard drives, replace both with SATA 7200rpm drives: one for the OS, one for games.
The stock Athlon 64x2 is overclockable, but not with the factory motherboard and not really that far (looked into it and found most only getting 2.4~2.5ghz). If you plan to overclock with a new motherboard, get an aftermarket cooler and a good quality motherboard (Gigabyte, Asus, Foxconn). You can find good Micro ATX sized motherboards with PCI Ex 2.0 for between $50-$80 on Newegg.com. That has always been my favorite place to shop in that they have good equipment, lots of info and customer reviews. Remember Micro ATX if you plan on using the factory case.......other larger sizes like ATX won't fit.
I've done a bit of research on this computer and have built/overclocked a few computers, so let me know if you want any help.
hey i got the same system and broke the power button which i need a new case to replace it so im looking into getting some things to help power up (beef up my system so ram and a power supply are big issues what about my on board 6150 grafix card should i upgrade that with a expencive model and do i need to replace both my drives or can i get away with buyng just one for gaming????