The Semprons are actually a mixture of rebadged Althon XP chips and indeed the Athlon 64 with the 64-bit part disabled. They are supposedly the budget range of chips as AMD move towards making the Athlon 64 range their mainstream line of CPU's.
Depending on what Sempron your sister has, you might be able to upgrade to a 64-bit CPU later on down the line, however from the sounds of it, I think the Sempron your sister has is actually one of the rebadged Athlon XP range.
Certainly I think a memory upgrade would be of benefit. The difference between 256Mb and 512Mb is quite noticable. The jump from 512Mb to 1Gb is less so. So another 256Mb of RAM would probably be the cheapest and most noticable upgrade.
Btw, a little know fact is that blade has an Athlon 64. I probably shouldn't have told you this as he likes to keep this fact a closely guarded secret. Cat's out of the bag now blade!!
