Yea, unfortunately GHz isn't a very good gauge of CPU speed - since Intel CPUs (like the i5) carry out many more calculations per clock pulse than an AMD FX CPU, due to the big differences in the CPU designs. Therefore, even at stock 3.3GHz the quad core i5 2500K is
significantly faster than the FX-4170.
If you do want an upgrade in the next few months, then I can recommend
this excellent £114 Gigabyte motherboard and for the CPU I would either go for a second-hand i5 2500K (hope to get one for ~£130) or this
Ivy Bridge i5 3570K. As you imagine, you can reuse your RAM and all your other kit.
Also, when choosing graphics cards you don't need to worry about going with a AMD graphics card if you have an AMD motherboard. Both Nvidia and AMD graphics cards work equally well on modern AMD motherboards like yours (in fact yours even supports Nvidia SLI), the same goes for Intel motherboards - Nvidia and AMD graphics cards work equally well.
Finally, please bear in mind that new 600 series Nvidia graphics cards (using the new 28nm process) are expected to come out later this month in your price bracket (the GTX 580 I mentioned is the top-end card from the previous generation, which has had some big price cuts recently). Considering the excellent performance, noise and power usage of the recently released GTX 680 (the first 28nm Nvidia card out) then newer, cheaper cards should be quite nice and give AMD a run for their money (we may even see some price cuts due to the new competition).