I was wondering if my low amount of ram is holding back my system in any way (I mainly play games). Resolution for the time being is 1280x1024 (hoping to go 1920 one day).
Also, would the increased speed of adding another stick (making it dual channel) be worth it?
I mainly play Oblivion and DIRT 2
2gb ddr3 1333
Athlon II x2 245 @ 3.6
Radoen HD 5750
The speed of your ram seems ok to me, so just add one more stick... Changing it altogether is very unnecessary... Two months ago, i had the msi 790fx-GD70 and i had my 2000mhz memory underclocked to 1333 because it gave me better timing and stability that way... So if your stick works fine, i won't advice you to change it altogether unless you have a desired better timing ram in mind..
You can go different brand but you have to make sure that the volts and the timings are the same. The timings can be looser on the new chips but the tighter timed chips will have to run slower too. The volts "HAVE" to be the same no ifs and or buts about it if they are not it could result in a very unstable system.