I'm also using an old Gateway P4. It originally only had 512mb RAM. I bumped it up to 2gb, then 4gb, and that's when I found out the 32-bit system could only use 2gb.
The best thing I did for this machine was to buy it an ATI 4650, 1gb graphics card. Where before I got maybe 10 fps with everything shut off, I now run it with all the graphics options maxed out, and it loves it. Funny thing is, it still has troubles with SH3 fully maxed. I would have gone with an even better card but my graphics slot is AGP, and the 4650 is the best I could by for that old slot.