The Sky Raider Mach II, LA Flyer, LA Racer, Super Stunts 40, and Super Sports 40S from World Models would all be terrific second planes. They are all available from:

http://www.airborne-models.com

Tower Hobbies has also picked up the new Thunder Tiger Tiger Sport 40L ARF:

http://www3.towerhobbies.com/cgi-bin...?&I=LXASPG&P=7

It's a very reasonably-priced low wing trainer and sport plane. I've always been very happy with Thunder Tiger ARFs, and I'm sure the Tiger Sport 40L will prove to be no exception.

Also available from Tower (in addition to the previously mentioned GP Super Sportster) are the GP Easy Sport 40, The GP Big Stik 40, the GP Escapade, the GP Cherokee, and the Phoenix Models Dolphin 40.

All of these planes, as well as planes mentioned further up this thread, would be a great next step after soloing on a high-wing trainer.

Good luck and good shopping!