Honestly, if your powersupply can handle a 4850, go for that. I have that on my desktop at home and it works like a champ. I replaced the old x1900xt that I had before and the difference was very noticeable.
If you aren't sure what it is, pop open the case and look at the powersupply.. It should have the watts listed on the PSU on a big sticker and you'll likely see additional plugs that aren't hooked up to anything. (Hopefully). If it is fairly low wattage wise, it's usually not that bad to replace one of those. You can get a decent PSU for around $50 (OCZ makes a good one). Just ask us if you aren't sure exactly what you need and whether the brand is decent or not.
Sometime in the next few months I plan on rebuilding my desktop with a newer processor (have a AMD 4800+ 64 right now) and moherboard so it can handle anything and everything i can throw at it.
And if you can, get Windows 7 in the near future.. It's great.
My pokemon brings all the nerds to the yard. And they're like, "You wanna trade cards?" Damn right, I wanna trade cards. I'll trade this, but not my charizard.
