Well, it's a 1st-gen i7 dual-core. The 2011 model is an i7 quad-core, with significantly faster graphics and Thunderbolt.
I'm seeing about $1100 used buy it now, on eBay for a similar model - just with i5 dual-core (10% slower, no big deal). So let's assume it's worth $1100.
They have a Macbook.. you have a computer they could only dream of being capable to build. If they wanted an Alienware with as much power, it'd be $2000. Out of curiosity, how much did your rig cost/ how much do you think it's worth? To me, it's fair to take current pricing of all your parts, and say the current value of the parts new is how much it's worth.
Anyways, bottom line, if you want a Macbook Pro 15 and you're not picky; it's not a terrible trade. I'd ask for a couple hundred on top of the MBP 15 to factor in time it took to build your system, plus imminent depreciation of their old Macbook Pro 15. Personally, I would wait until the 2012 Macbook Pros come out, probably by/before June. Then you can pick up a 2011 model for $11-1200. I would go with a different system if you just want something nice/not necessarily a Macbook. You can get a sick gaming laptop for $1000, or you could get a new HP Envy 15 for about $1200, with specs as good as the current MBP 15 (except the Envy has no Thunderbolt, has a 1080p screen vs. the MBP's 1680x1050). Or a Samsung Chronos 7 for about $1200 new.
But yes, 330M should do LoL fine. My Intel HD 3000 graphics can as well - 330M is a little faster.