A 128-bit FX5200 is still pretty good for modern games if you don't use AA/Aniso, but the 64-bit version has performance closer to a GF2mx or a 9000se. If it's a 128 bit card, it should run almost any game at 1024x768 at acceptable framerates (40 fps or so). It's roughly equal to a GF3 (non-ti), so it's not all that bad, it's just that there's much better options out there (though it's also the cheapest). If there's a noticable performance differance between it an the TI200, then it's almost definatly the 64-bit version. Try to pick up a 128 bit card off of newegg.com, you can probably pick one up for around $70 shipped. Just remember, getting a video card with 256 megs of ram won't really help, 128 (hell, even 64) megs is more than enough for today's games.
But yeah, ram is your main concern. You're probably noticing a lot of pauses and such in the game, and that's because it's using virtual memory like mad. Try to get more ram on sale, I've seen 512 meg chips go for $70 or so after rebates, so you should be able to pick up both the ram and a decent FX5200 card on your budget (if you need to buy the ram first and then get the video card when you get your rebate back). Kinda funny though, you have a video card with as much ram as your computer
And you shouldn't have any problem returning it to Best Buy, as long as you have the original packaging and reciept (and it's within 30 days of the purchase price).