Unsure about buying a new PSU

First off, I built this computer a little over 8 months ago. The motherboard (ASUS A8N-E) has the 24-pin power connection, but my PSU only had the 20-pin (the case is almost 4 years old). The Antec Power Supply gives out 350W (I think), and after reading around online, I just plugged in the 20-pins and went with it. The machine has been working fine this whole time, just some random slow downs in games, that I'm not sure should be happening... it doesn't seem to slow down when it should, like when there's tons of stuff going on, but also times when nothing is happening. A few games will just chug along for a few seconds, and then start working fine again. And then 20 or so minutes later, do the same thing. I just don't want to be intentionally handicapping my system instead of buying a 60 dollar power supply.

Also, from what I've read, the computer shouldn't even be booting since I have a PCI-E card in.. a 7600GT. I was going to just buy a new PSU, but the only brand I was familiar with was Antec...and almost every Antec PSU on newegg was getting slammed in reviews for being faulty and failing.

So, instead of buying a new PSU now, I could just wait about another year until I build a new machine and overhaul everything. But, I'm wondering if I've just been pressing my luck this whole time, and one day my computer will just decide not to boot, or worse, fry everything. I was going to buy a 500W supply (that I would keep around until my next machine) but if the PSU isn't a problem, I might just keep waiting.

To sum it up, should I bother buying a new PSU with a 24 pin connector? Or should I just keep the one I've got, since it seems to be working?

And any recommendations on a PSU brand would be helpful...has Antec really dropped that much in quality? I checked the sticky but couldn't really sort through all of the supplies. I'd be looking for something around 50 bucks and 400-500W. I don't SLI, but would want the new PSU to be able to handle a few SATA drives, a DX10 card, and a dual-core processor when I finally upgrade.

Any help is appreciated.