The radio that comes with the Slash is a 27mhz AM and you only have 6 frequencies to choose from. You could go 75mhz and have 30 frequencies to choose from... but if you are going to upgrade, you might as well go 2.4ghz and not worry about frequencies at all...
I had used 27mhz for over a decade.. racing too... it got to be such a pain at the track because most of the RTR cars started coming out with 27mhz, it was so difficult to get any track time. About 4 or so years ago when the 2.4ghz came out, it was a godsend! I haven't looked back since!
With a 2.4ghz, you "bind" your radio and receiver(s) together and they find an open "channel" within the 2.4ghz range and are encoded. It is extremely rare for you to have any problems.. if you do, you just turn them off and on again and they find a new chanel.. but like I said, it is EXTREMELY rare.
You can find these radios for just over $100 and top of the line for about $200 nowadays.
Even the lower end radios are exponentially better than the basic Traxxas radio you get with the slash. Tons of trims and adjustments as well as several car settings for multiple vehicles. Even if it was just for the 2.4ghz it's worth it for the amount of track time you gain without any headaches!