Depending on the unit it could be either a slight difference or a relative significant difference. If you get a decent head unit it will also allow you to tailor the sound to your liking.
i'd look at a pioneer or kenwood mid grade head unit, from 150-250 depending on if you find one new or used. As far as speakers i personally would never run anything but components for my front stage, and there again is alot of options, (but to get the full potntial you'd want to amp them up). I'd look around at some user reviews on the site here and see what you like. Right now everyone is liking hertz and morel which are about 250 for your basics. A while back it was all about phoenix gold pgrsd's which you can find for around 75 on ebay. When it comes to head units it really depends on what features you want, ipod, bluetooth, active, eq options, etc... In my personal experience i wish i'd have listened more and saved more money before purchasing my gear. i've had to rebuild a few times and spent more than if i'd have just saved and bought the good gear the first time. Budget systems in my experience usually sound like budget systems...