So.. Basically you need a CPU upgrade if you want an improvement, and with a CPU upgrade more stuff follows. You're looking at a new motherboard and new RAM as well. I would suggest a "bang for the buck" upgrade for about £200-300 where you upgrade these key components.
Well then. Not to be pushy but I will give you a list of price worthy components, and I'll be open minded to both AMD and Nvidia. I'll give you not one, not two, but six builds with a motherboard, processor and memory modules. Keep in mind that I check prices at Swedish sites
Here goes AMD:
£369.33/575.83 USD
AMD Phenom II X4 955 Black Edition, Quad Core, 3,2Ghz, AM3, 8MB, 125W, Boxed
Gigabyte GA-790XTA-UD4, Socket-AM3 AMD 790X+SB750, ATX, DDR3 , USB3.0, SATA 6Gb/s, GbLAN, 2xPCI-Ex(2.0)x16
Corsair Dominator DHX DDR3 1600MHz 4GB Kit w/2x 2GB XMS3 DHX, CL9-9-9-24
£326.33/508.80 USD
AMD Phenom II X4 945 3,0Ghz, AM3, 8MB, 95W, Boxed
Gigabyte GA-MA790X-UD3P, Socket-AM2+/AM3AMD 790X+SB750, DDR2, ATX, GbLAN, Firewire, PCI-Ex(2.0)x16
Corsair TWIN2X PC6400C5DHX DDR2 4GB KIT Kit w/2x 2GB XMS2-6400 dimm's, CL5-5-5-18, 800MHz, DHX
£188.75/294.28 USD
AMD Athlon II X3 435 Triple Core, 2.9Ghz, AM3, 1,5MB, 95W, Boxed
Gigabyte GA-870A-UD3, 870, S-AM3 AMD 870+SB750, ATX, DDR3, USB3.0, SATA 6Gb/s, GbLAN, PCI-Ex(2.0)x16
Corsair Value S. DDR3 1333MHz 2GB CL9 Unbuffered, CL9-9-9-24, 240pin, 1.5V
Okey AMD, you're out, let's get on the awesome Intel train.
£369.33/575.83 USD
Intel Core™ i5 Quad Processor i5-760 Quad Core, 2.80Ghz, Socket 1156, 8MB, 95W, Boxed w/fan
Gigabyte GA-P55-USB3, P55, Socket-1156 4xDDR3, ATX, GbLAN, USB3.0, PCI-Ex(2.0)x16
Crucial DDR3 1066MHz 4GB KIT, CL7, Kit w/two matched DDR3 2GB, 240pin
£343.53/535.61 USD
Intel Core™ i5 Dual Processor i5-650 Dual Core, 3.2Ghz, Socket 1156, 4MB, 73W, Boxed w/fan
ASUS P7P55D-E LX, P55, Socket-1156 DDR3, SATA 6Gb/s, USB3.0, ATX, GbLAN, PCI-Ex(2.0)x16
Kingston ValueR. DDR3 1333MHz 4GB, CL9 Kit w/two matched ValueRAM 2GB DDR3
£236.39/368.56 USD
Intel Core™ i3 Dual Processor i3-530 Dual Core, 2.93Ghz, Socket 1156, 4MB, 73W, Boxed w/fan
ASUS P7H55-V, H55, Socket-1156 DDR3, ATX, GbLAN, HDMI, PCI-Ex(2.0)x16
Corsair Value S. DDR3 1333MHz 2GB CL9 Unbuffered, CL9-9-9-24, 240pin, 1.5V
Anyway, good luck with that!
Edit* I just added the real prices instead of my estimates of how much a quid was worth
I also added dollars, it doesn't say on your profile where you're from..