So I am looking for a GPU and I have around~330-430 to spend.
I had originally chosen this one-
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Gigabyte GeForce GTX 670 Windforce 3X 2048MB GDDR5 PCI-Express Graphics Card £339.95
Total : £350.75 (includes shipping : £9.00).
But my question is, for a little extra am I able to get a much better card? Or is that one pretty decent and more would just be spending more for a little improvement?
Now to answer the OP: can you get a much better card? No. The 670 is within 5% of a similarly clocked 680.
SLI/Xfire isn't just for multi-monitor. It can push you closer to the refresh rate of a 120 Hz monitor but you would be wasting your money a little if you had a 60 Hz monitor unless it's at a resolution of 2560*1440 or above.
To give you a rounded view rather than some petty bickering which you'll see above, the 7950 offers the best price to performance ratio in the high end by a good margin. At a max OC it'll be a little bit faster, even, or a bit slower than a similarly max overclocked 670 depending on the game. However, you are reliant on getting a fairly decent clocker. I don't know what the chances are of getting an overclocker which matches a 670 but I would say it's a good chance but not certain by any means.
Depending on the game as well you won't reach 120 FPS with only 1 graphics card so you can look at multi GPU's of your motherboard and PSU allow.
Again value for money first would be 7850 crossfire at the low end and 7950 crossfire at the top end. You wouldn't be able to get a decent SLI set up for your budget.
Some people prefer a brand of graphics card over another or want a feature one has which the other isn't able to run without a large hit on performance. An example would be AMD's advanced lighting in say DIRT Showdown or nVidia PhysX in Borderlands 2. If you have a preference of drivers it can often be a reason to stay with either manufacturer.
If you've got a blank slate then I personally would go 7950 crossfire. If you want a single card then I would weigh up everything and probably get a single 7950 however a 670 would be close and depending how I felt I would go this way!
The Gigabyte Windforce is the best 670 IMO.
Is the 670 a better all round card than the 7950? Extremely marginally but you have to consider the price difference of £60-100 depending on deals and say it's just not even close to being worth it with a blank canvas...
Hope that helps - any questions feel free to ask.