^same here, played with the Intel 4600 in my i5 4690K and HL2 runs very smoothly, even at its highest settings, although it can suffer slowdowns with AA cranked up.
As for a dedicated card, anything from the 8800GT upwards (with 1Gb of memory if you want to push the AA to max) will do the job more than perfectly: http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32899-7.html
With all settings on max without antialiasing I can easily get a solid 60fps on my Intel HD 4600 on Counter Strike Source. On Dota 2 I would imagine you could get anywhere within the 30's on decent settings with an integrated GPU.
^same here, played with the Intel 4600 in my i5 4690K and HL2 runs very smoothly, even at its highest settings, although it can suffer slowdowns with AA cranked up.
As for a dedicated card, anything from the 8800GT upwards (with 1Gb of memory if you want to push the AA to max) will do the job more than perfectly: http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32899-7.html