GF4 does MSAA, like GF3. GF2 and previous do SSAA.
irenealan, AA and AF are basically image quality enhancements that the video card can provide on top of the game's visuals. They exact a penalty in framerates, though, so you're trading off speed for a little nicer quality. They're certainly recommended if you can afford to turn them on, but at the price level you're discussing, really not usable, as the 4200 and 9600 are both relatively slow to begin with in newer games. Generally, you want to aim for 60fps (frames per second) average for a fluid framerate that won't hinder your gameplay. Once you dip into the 30s, the game will still be playable, but there'll likely be moments of noticable slowdown, when the framerate hits the teens or lower and your control becomes noticably choppy. Thus your first priority should be fluid framerates, with nicer IQ as a secondary (but obviously desirable) consideration.
Here is an article with a comprehensive array of video cards benchmarked on a wide array of modern games. (At the end, you'll find benchmarks conducted with AA, AF, and AA + AF, and you'll see those settings exact a generally large performance penalty at the low end, where you're looking.) Find the video cards you're considering, and see how they fare against each other in the games you intend to play. Then weigh their performance against their price, and you can arrive at the card that offers the best prospects for your games at your budget.
Leadtek is a good company, and the 5700 is a decent card, but I'd recommend a 9600 Pro over a 5700 at the same price, simply because it's faster overall (particularly with AA &/ AF, as the benchmarks indicate). But you'll also notice that the 4200 is pretty darn competitive (aside from with AA &/ AF, which are "luxury" items), and thus a better bargain at $80-100.