My PC is not any better than this, I only have a dedicated GPU (6150SE is an integrated card if I remember well). So the cheapest solution for you would be to get one of the cards
recommended. Anyway, with a setup like that you should be able to play in 1920x1080 with details on medium (AA and AF off, ofcourse), I'm pretty sure about that since my setup is pretty much the same, only I'm on 8800GT 512 MB GPU, there's nothing new, expensive or impressive about my PC.
I usually get 30 fps outside and 50+ inside, it only falls under 20 in rare situations (too many models on screen, etc etc) but that's rare so I can say it runs pretty smooth 99% of the time. So if you don't mind the fact that you won't exactly be playing in "trailer quality", go for it.
I can also post screenshots with FPS if needed.
EDIT - Also, I have 3 GB of RAM, just remembered, I don't know how much that helps with the "smoothness".