For me, fermentation temperature control (following a specific profile) was the single largest quality improvement to my beer making over the last year. Of course, some styles and yeast strains are more sensitive to temp changes, others are more tolerant. I've been making mostly ales and the occasional stout lately. Pitching the right amount of yeast at the initial target temp. was also significant as it seemed to get the fermentation active within 6-12 hrs after pitching, complete a lot faster, with much cleaner flavors.
That said, there is a lot you can do with little investment. Using a water bath around the fermenter that is held at desired temperature works very well. Add ice (or warm water or use a cheap aquarium heater in the bath if you're fermenting in a cold basement) as needed during the most vigorous stage of fermentation (first 2-3 days or so). In the early days, I also used a wet towel and a small PC fan to cool the fermenter. Works well, but you need to monitor on a daily basis.
Good luck!