sandy bridge. there is the 2600 and the 2600k. both are quad 3.4ghz processors that turbo up to 3.8ghz.
the 2600 is a locked processor and it cannot be overclock. in actuality it can, but only up to a max of 4.0ghz and only if you have the right motherboard.
the 2600k is an unlocked processor and along with its little and big brothers the 2500k and 2700k(all originally cast the same then binned out), are arguably and probably the best overclocking enthusiast cpu die ever made. with decent water cooling like a d5 pump and xpsc 240 rad, a 2600k will easily hit 5.0ghz fully 24/7/365 stable, and can regularly be pushed into the 5.3-5.6ghz range for benching.
for gaming yes, the regular 2600 is more than fine, but it lacks in the reasoning that if it were a 2600k, all you would need is a cheap $25 air cooler and you could happily overclock it to 4.5ghz which is about a 35% gain in performance over a regular 2600 which is very considerable.
if your not comfortable with overclocking, then dont worry about it... an i5-2600 and an r9-270 will give great 1080p gaming and is a good match up. the only thing you should look at is the power supply in that computer as it may not have enough juice to power the r9-270 and the rest of the computer, so you might need to invest in a decent quality 500-600w power supply.