Gaming - ultra specs

i have a late 2011 15" mac book pro, 2.5 Ghz Quad intel core i7, 8gig 1333 mhz DDR3, AMD Radeon HD 6770M 1024 MB, Intel HD Graphics 3000 512mb and a 256SSD... yet my comp lags when i play high - Ultra settings on (the witcher 2 enhansed edition)... i don't know how to improve performance correctly. Should i Upgrade th graphics card? get more ram? asking for some advice?

sorry about the bad grammar and spelling.