As long as your turntable doesn't rumble it's better to put your cash into buying a better cartridge. Make sure that the arm is balanced properly and that you've got the tracking force set to how you like it. The only advantage a mixer will give you is outboard EQs and a crossfader for chopping. It's sometimes useful to have a different choice of EQs/filter if you want to get a more analogue filter sound.
If you've got the cash, I'd recommend getting hold of an Ortofon cart and stylus
http://www.needledoctor.com/Brand-Store ... -DJ-Stylus