Try the graphics card. One with at least 1GB of vRAM on the card would do much better for today's games.
Reformatting isn't going to fix your problem. That's just a temporary solution, and it only works for a short time because of the inefficiencies of the Windows filesystems. They slow down over time, and so do hard drives.
That's still not gonna help with your overall performance issues, if it is a video issue. Which it is.
Your RAM is more than enough. 6 GB can more than handle any game out today. You shouldn't have to play this game on normal, or low. Just upgrade the card then you can play on highest with no lag. Don't have to worry about technical crap you couldn't care less about.
But uh... get an NVidia card. ATI cards are super suck with Vindictus especially. Their drivers are just... bad. They haven't honestly even been competitive with GeForce for some time now. It's kinda like Intel vs AMD. One's cheaper, but... nobody should really recommend it.
I got
this April 20th. It was $247. Should still be priced similarly.
The sewers do seem a bit harder on systems though. I even get a little bit of lag on my end when I host sometimes in the sewers, running on highest settings. It's just slight though.
Honestly, I think they just did a crappy job making it. Kinda like Age of Conan did where even the most expensive systems couldn't run it for crap. I mean, I'm running an i7 920, 12 GB of RAM and a GTX 560 Ti. I shouldn't lag in anything. It can pretty much chew up and spit out any game on highest.
But still... the sewers. I dunno what it is about them. Maybe all the reflections on everything or something...
Do you lag during any of the other ones, or is it just the sewers? If it's just that, then honestly I think it's probably their problem.
I doubt they'll fix it though. So, a new card would probably be a good idea for now. Reformatting isn't gonna help you. That's not even a good idea.