eh....couldn't really give you recommendations without knowing
A)what you need to do with the box and
B)what the box's specs are

If it's too slow for what you're doing, you could need a new proc,memory,or other parts. Then again, you might have a decent CPU and need more memory. If you don't have enough memory, your computer uses to hard drive for memory which is waaaay slower. Then there's also the issue of how old your mobo is. Or it could be something else. Like my ancient ATX PC with a massive toggle for a power button....ok, not really, but is IS old-it has the reset AND turbo buttons.....I'd love to upgrade it (more than I already have, it's 8 or 9 and running XP alright, so you could probably guess I've done everything to it) but you can't take a nice shiny 3.6 GHz pentium 4 with hyperthreading and put it where your 500MHz p3 used to live.... :)

If you screwed together your computer from parts into a good case, then you can often upgrade parts at a time. If you bought the computer from a vendor (Dell, HP, Gateway, etc), then it's much less likely you can upgrade parts of the computer (if nothing else then because the power supply is usually weak), so you should buy an entire new computer.

If you want to use your computer for heavy-duty recording, 3D creation, competetive gaming and software development, you probably want a dual-core CPU (Athlon 64 X2 or Pentium D). You also want a fast hard drive (Western Digital Raptor), and a nice grahpics card (GeForce 7600 GT or 7900 GT). I would recommend 2 GB of RAM these days. I recommend at least 500W power supply. If you're not familiar with electronics, then buy it online from a good vendor (Newegg.com comes to mind) or from a systems builder (like Dell); else buy the parts and screw it together yourself (don't forget case, power supply, DVD burner, and copy of Windows).

If you are not a hardcore gamer/developer like that (or cash is lacking) then buy anything that has Radeon or Geforce graphics, and a fair bit of RAM (at LEAST 1 GB; prefer 2). Avoid Radeon Express graphics, though. Maybe a Dell E510?

If you need a new display, buy a 20" flat display with 1920x1200 resolution -- the 1280x1024 are on the way out, and don't have enough pixels for the future.

I would definitely upgrade the RAM to 512mb or 1 gig at least especially with WinXP. Everything else really depends on what you need it for.

Well, if the OP is not a hardcore gamer/developer, memorywise 1G should be plenty. Maybe even 768MB.

To the OP: The 256MB of RAM is probably killing you, though. If you go to crucial.com, you can find out exactly what memory is best for your computer (I wouldn't buy it from crucial though, they're pricey. Try tigerdirect or newegg)

I don't know anything about the processor, there's usually a number after the "Athlon XP" part that you need to know to find out what speed it is.