I'm using a first-gen i3 (@ 4ghz) since release, and recently paired it with a 7950. So far there's no 'casual' games I can't max out, and I get decent frames with lots of eye candy in *many* modern games.
When you read the system reqs, most modern, non-casual games require 2 cores around 2.4ghz, and recommend 4.
Sometimes you might have to turn down CPU intensive options. Things like ragdolls and other physics, AI traffic/people density, etc.
Some games are good at telling you which options are CPU intensive and which are GPU, others it's just trial and error.
I have a SONY VAIO VPCEB36FF with :
1st gen core i3
4gb 1333Mhz RAM
ATI radeon HD5650 <==
Full HD screen <==
I played Crysis 2 with full HD and medium details at around 30fps, and I play all the NFS with full HD and high details with really good fluidity
It is the graphic card that really counts, so go here : http/www.notebookcheck.net/Mobile-Graphics-Cards-Benchmark-List.844.0.html
and check whether your graphic card is in class 1 or 2, or if it is class 3 it should be better than the intel HD4000 I would say. That's all you need.
One last thing, if you have an HD screen, even the intel HD4000 is enough for casual gaming.
If you click on the graphic card in the website I gave you and then scroll down you'll have a list of games the graphic card can play and the fps for each game vs details level.
it is more than enough for any gamer
gddr5 is always better choice, it is just that having low definition such as 1366*768 doesn't need such performance to play games, but more performance is always welcome
one last thing, I would personally prefere having a samsung ssd 840pro 250Go than a classic hdd at 1To, it will lower loading times (but will not affect game performance)