Well since you have a XFX you won't yield very high overclocks. Reason being they use cheap parts that barley run at stock speed, that's why their cards are considerably cheaper. You have to downclock your card slightly to get it running properly, if you'd like to go any further you might want to consider better cooling, to replace the stock heatsink and fan. Some else can reccomend a good cooler because I can't think of one of the top of my head...sorry but I sure hope that helps!
EDIT: I think that this
one will work on your video card.