it's a very good policy to match the new memory brand and type with your existing make'n'model. Then you know they'll play nicely together. It's not all that common for two different brands to cause issues with each other, but it does happen sometimes.
Another good reason to match is simply that not all memory works with all motherboards. Don't let that make you paranoid, unless you're working with a very old PC or buying very old used ram from ebay or something, you probably won't end up with something that simply doesn't work
Also try to figure out if your memory is DDR-2 and if so, how it works on your system. Basically DDR-2 is a way that two sticks of memory can join forces to form voltron and be super awesome memory, but for it to work the sticks usually have to be a good match... same brand, same size, and sometimes they have to be in certain physical slots on the computer's motherboard. Most modern computers accept (and come with) DDR-2 capable memory sticks. If you aren't sure, get help and check because DDR2 won't work in a system made for DDR1 (or older) memory.
As for favorite brands or recommendations... you don't HAVE to stick to the same brand as whatever's in there, and different brands of DDR2 memory can sometimes still join together and do their thing. So if you buy a different brand, I'd just check the newegg ratings and trust those. If like 200 people give it a 5 star rating you're probably safe getting that brand.