if OPs card is a HD 6570,trying these flags might help:-v -w128 -f1
Type this in the flags section of GUIMiner as written.
Also using the latest version of GUIMiner can help boost performance due to optimisations as well. Overclocking that card (core clock) should help boost your hash power.Memory clocks should be lowered due to the fact that Bitcoin mining makes little use of how fast the memory is running at,it's more to do with the GPU itself.
Even if you do manage to squeeze some extra performanec,the hash power you get isn't really worth it unless you have free electricity.If you are able,get yourself a better GPU.Look at the list of popular configurations on that mining hardware comparison as it'll tell you which cards are particularly good at mining.If a card is not in that section,it's probably because it isn't the best option for mining due to the hashpower they give for the price,or they use a lot of energy for the hash power given from that card.
I hope this all helps you (and all the replies here).