First off, without knowing the speed of your CPU it is difficult to say what exactly would be best to run with it. It looks like it is a dual core, but with it being a Phenom II it should be mounted in a socket AM3/AM3+. If this is the case, it shouldn't be that tough to upgrade the CPU if you need to at any time.
As for the graphics card, that system should do well with any of the newer cards as long as you don't get one with more than 1Gb of ram on it if you are running 32bit version of Windows. In 32Bit windows you already have more ram than it can handle so having more graphics ram would reduce the system ram by that much more. If you are running 64bit version of Windows then this does not apply.
As for brand and how things perform, you are right there are several brands out there with the same basic card. The differences could be slight, or non-existant or they could be large depending on what the manufacturer decided to do. They will all perform the same, but there will be things like games included or a graphic port not added or different fans and shrouds. Have a look
HERE and look in the price range you are looking at to see what Tom's suggests for your price range. This site has been around for quite some time and I have seen that they are not biased and are not paid by companies to promote product. So if they say it is good, they you can usually believe them. For the brands, find the brand that offers what you are looking for in the card. If you want the crossfire or SLI bridge, make sure there is one listed before you buy. If you wish to get additional software, choose the product that has the game or what ever you are looking for. In the end performance is going to be about the same regardless of brand, and from what I have seen, pretty much all companies are good with their support at the present time.
Finally, as to how the games will perform, there are many different factors that will determine this. The CPU, ram, GPU, system software, game software and internet speed if it is an online game. Game performance is usually linked to a single core of multi-core systems at present, but there are games that can take advantage of all available cores. With this being said, the faster a single core on the CPU is, the better the games will like it. GPU performance has a large effect, but going with crossfire or SLI may not change how the game performs due to issues with SLI and crossfire. On single card setups if you have 64bit Windows the options are large, but 32bit Windows has some very large limitations so be careful.
