@ Orestes77: Must have missed your original question.
I'm fairly sure the XPS series will allow overclocking (yes, they probably said that
), it's a very good idea, the i5 seems to love being overclocked, mine runs happily at 3.4GHz on a fairly low end Coolermaster Hyper 212+ with no issues. And I know-I'm not even trying.
There is plenty of cards that are a decent upgrade:
http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32716.html
I'm running a HD7950 on a single 1080 monitor and see very similar numbers to the review figures for my card, despite the fact most reviews use more powerful, more heavily overclocked CPUs.
All of which underlines the post by sam_p_lay, yes it's a little old, but even the most cutting edge games get a fairly small boost from the most modern chips.
Given the current prices, I'd jump all over a GTX760-HD7950 performance for <>$250 with improved thermals and a quieter cooler? No brainer really (unless you want the free games from AMD-and can get them
).