OCZ, corsair, patriot, kingston, and mushkin are fairly popular brands. They each have their own levels of high end and low end series, much like intel has the Pentium and celeron series. Most popular is the Corsair ValueSelect series at the moment.
What make is the 512mb you currently have in there? If you plan on continuing to use it along with a 2nd stick of 512mb, it is highly recommended you get the same make if possible, if not at least get the same speed ram. What is your current ram? PC3200 (400Mhz)?
You might as well try to search yahoo or google for Infinion 400 MHz RAM then. If that particular brand is not available, then you can still buy different brands, but at the same speed.