If your unsure about your card check this site out.

http://www.hwcompare.com/5617/radeon-hd-4870-1gb-vs-radeon-hd-6750-1gb/

choose your card vs the recommended cards
4870 or gtx270

Or use Anandtech.com which is a full coverage hardware site with cpu and gpu charts plus lots of in depth
articles where you can learn things you did not know before.

http://www.overclock.net/graphics-cards-general/502403-graphics-card-ranking-5th-time-last.html

Updated regularly.