A good quality 300 watt will still get you by but i would suggest go to 350. There should be one or two of those power supplies that are under $50, more like $40.
Do you have any sata drives?
If you plan on rebuilding your system in the future and may want to keep the power supply, then you might want to plan for that and get a ps that has sata connections on it as well, just a thought. If you plan on keeping th new ps you buy with that pc and sell it when you want a new pc, then all you may need is a plain, good quality atx ps.
ps=power supply.
To be sure, give us the info on what you have installed on your current pc, the mobo, cpu, ram, hard drives, optical drives (cdrom dvdrom, burners), video (its above but it would be best to have info all on one place), and whatever other hardware you have on your pc.
Also, you might check the specs on your vid card to see how much power it drains, or check the video mfgrs website for info on that vid card, check their FAQ section.
That way we can give you a better answer on the ps. Too bad the $25 unit rebate ended, that was a good one. Another will come up though, always does!!
I take it your current power supply main connector is 20 pin, not 24 pin?