Many phono preamps over $100, and some under $100, are likely to to be an improvement over the internal phono preamp in the Yamaha R-S500.

http://www.lpgear.com/category/SS3.html

If you want to upgrade what would be your budget?

Nagaoka MP-150 - tops my list

Ortofon 2M Blue - well regarded, I thought it a little harsh

Audio Technica AT440MLa - I have this one on my Pro-Ject xPression III, needs appropriate loading to not sound overly bright, very good tracker, $174 at turntableneedles.com

Goldring G 2100 - Well regarded, but it is not the 2200 which is very good

Grado Gold1 or Silver1 Prestige - could be a good one if you like the Grado sound

Ortofon MC1 Turbo - very nice high output MC for the money

Denon DL-110 - only $140 but hits well above its price

VTA and tracking force are also important. You might want to check out Vinyl Engine for more turntable resources.

http://www.vinylengine.com/turntable_forum/index.php