You'll have no problem getting it to run smoothly. It's a very well coded game. I play it on 2 vastly spec'ed PCs and it runs great on both. On my desktop I have a quad core, 4GB, GTX460 @ 1280x1024 and it is silky smooth with everythin on high. On my laptop - Dual core, 4GB, 9800GT @ 1440x900 and it's smooth with it set to medium/high. You won't have any problem running on high at that res.
As for modding, this game is very easy to mod. I've done the black market mod (opens up all items), superman mod (changes you to be a flier without a chute), costume changer (let's you wear the outfit of any NPC, Ninja's my favorite) and massive explosions mod (makes you C4 like a mini nuke, very fun)
My copy is from steam and it doesn't mind. If you go to drop the files in the folders, you just have to go to Steam>Steam apps>common>Just cause 2 and most of the stuff goes into a folder called "dropzone" Very easy