If you're not too picky about graphics, it will look fine. Because it will be so much bigger you'll see all the jaggies and interlace artifacts from blowing up 480i. Some games can run in 480p, but you need component cables for that.
Also, be aware that almost all PS2 games have significant underscan built it -- this means that they will NOT fill up the entire screen of the plasma, even if they have widescreen support. You will usually get a 1" - 2" black frame on the sides, and sometimes a little bit on the top and bottom too. You can't get rid of this by using the TV's stretch modes, it's built into the signal.
If you can deal with that, the games really do look great and play the same (but bigger!). If you're not a graphics snob it'll be fine, but you'll probably start itching for a next-gen console soon.
I sold my PS2 and grabbed a cheap used xbox instead. If you get the component cables pretty much all xbox games support full 480p, and many good games have widescreen support and sometimes even 720p. And the underscan issue, while still occasionally present, is far less common. IMO if you aren't a serious enough gamer to want to spend $300+ on a next-gen console, the old Xbox has a lot better graphics than PS2.