I'd hold off on a new PSU for now, the Thermaltake can handle either of those cards. It wouldn't be worth upgrading now unless you can spend at least $75 on a new one.
That exact ram is discontinued. Another pair of 2x1 may or may not play well with what you have. Current price on DDR2-800 2x1 CAS 4 is in the $40 range and 4 modules at 2.1 volts may be hard on the motherboard. For $55 you can pull it and put a 2x2 kit in:
Newegg.com - CORSAIR 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Desktop Memory Model VS4GBKIT800D2 G
XP will recognize about 3.25gb. Note that any DDR2 ram is dead-end, you won't be able to reuse it with any motherboard upgrade.
My grandkids are all gaming with 2gb and XP.