The longetivity of the PSU doesnt really matter in this case.
It just matters if it is made of good parts (which I highly doubt it is) and has enough amps on the 12V rail (since this is what powers most of your components in your computer).
You most likely will have to replace that as well.
Concerning the RAM issue.
If you have the money why not just upgrade both computer's RAM.
512mb is not very useful anymore and will def. bottleneck your computer.
I suggest either getting a 1GB module or 2x 512mb modules.
I usually lean towards getting a 1GB module just in case you want to switch to 2 x 1GB modules.
The reason for this is bc in older computers, the motherboards will actually make the memory run slower if all 4 DIMMs are filled with memory (for example 4 x 512mb).
However, if only 2 DIMMS are filled (i.e. 2 x 1GB or 2 x 512mb) then the memory will run at the best speed.
RIG #1 - See System Specs.
AMD X2 3600+ (AM2), Biostar TF550, 2 x 1GB Buffalo Firestix, XFX 9800GTX+, WD 250 GB HD, NZXT Nemesis Elite Case, Lite-On Combo Drive and Lite-On DVD RW Drive, Antec TruePower 550W,
20.1" Dell w/ 17" Sony LCD