Need advice... power supply

I'm working with bare minimums here so don't laugh...

Ok I've got an Acer aspire 6600 series, it's running with a 500mhz Celeron processor and 375 sd ram. My power supply tops out at 145w, and what I need to know is what kind of power supply to upgrade with. I've checked out ebay and there were a few lookers. I'm trying to get it up to 250w but I don't know if I have to use I certain one or not... this is my first time even caring about the power supply so any help on that manner would be greatly appreciated.

I'm trying to upgrade it so I can get a decent video card... as you can tell from the processor, I'm not looking towards the high end of the field. I just need something decent and cheap. There were a few at wal-mart but if anyone has a better idea that would work too... I just need something that can handle a N64 emulator and better streaming support. The emulator I'm using now runs like crap because of my specs... it's still suprising to see what it can pull off though. At the moment I have the bus set up at 60/100 instead of 60/60 and it's managing pretty good on the processing side. The emulator's system requirments were a 600mhz proccessor and alot of other things I didn't have but it didn't stop me.

There's a cheaper solution that envolves going out and buying a N64 but I just want to get the overall system stability under control... just watching Flash based movies like youtube are a pain in the...

If anyone could tell me about the power supply it would be nice