Don't forget some ENB's require other mods to run, such as the RWL mod and the NVSkies mod. You can search out a decent amount of them on the Nexus, and some nice ones on here too.
As stated above, your PC seems mostly adequate, it all just depends on what else you are running along with the game. Lag can also come from mods and mod conflicts as well, so be cautious of your load order and use a good mod manager (FOMM and FNVEdit help a ton for me, also I hear LOOT is good too).
Check Me Out @ The Nexus: SKYZOO