What graphics card would be best for my computer to play starcraft 2?

I pust bought the following computer, (http://www.bestbuy.com/site/HP+-+Pavilion+Desktop+/+AMD+Athlon%26%23153%3B+II+Quad-Core+Processor+/+6GB+Memory+/+1TB+Hard+Drive/2670319.p?id=1218343206257&skuId=2670319)...the only difference is mine is the 640 instead of the 645 processor.. I can play starcraft just fine on low settings but any higher and i recieve lag. It says "ATI Radeon HD 4200 integrated graphics Feature up to 2943MB total available graphics memory" i know integrated sucks for games but shouldn't this still be able to play starcraft on med-high graphics? is there a way to dedicate more memory to starcraft? If not what would be some options to upgrade my graphics card....i'd be willing to spend around $100, hoping to be as cheap as i can without losing quality...thanks for any help