The X300SE is a terrible card. Infact, the X300 series is pretty bad. Stay away from anything ATI with the SE tag, (and same deal for nVidia with the XTs).
You really need to label down your budget, and I personally hold the opinion that if you can only afford one of those two cards right now, you should hold on and save a bit more.
In terms of new generation, the X300, X600 and X800 are not new generation, just faster versions of the current gen (especially X800). The Geforce 6's however are new generation cards, with new hardware feature support like SM 3.0.