Alright Ive been wanting to get a new system for a long time but haven't had the money. Some good fortune has come my way and I've got a little bit to play with. I'm a bit of a noob when it comes to all this, hence why I'm asking you guys for your advice 
I'll just post my current system here if theres any chance of upgrading it (which I highly doubt) oh and don't laugh please I've had it for a good while
Some sort of Win Power? 500watt power supply (yeah I've never heard of them either)
Foxconn 6150M2MA-KRS2H motherboard
AMD Athlon 64 X2 Dual Core Processor 5200+ 2.61 GHz
Corsair Value Select VS2GB667D2 2GB of ram
ATI Raedon HD 4650
Hitachi HDT721010SLA360 1TB Hard drive
SAMSUNG HD501LJ 500gb Hard Drive
Monitor is 19" and the resolution is 1440 by 900
Anyway yeah thats that, ideally I was looking to build a new system though and keep my 2 hard drives and monitor (and I also wouldn't need speakers, mouse, keyboard or OS)
I was looking at the i5 and Phenom x2 955 processors and they looked pretty nifty. (Would I notice a big difference with these compared to my current processor?)
Would like a 5770 if possible too.
I would also like to be able to overclock my processor too eventually (I will figure out how to do this one day!)
That's all I really want, I don't really know much about the other hardware like the RAM, CPU cooler and motherboard.
Can you guys recommend me a build (keeping in mind what I said above) that is in my budget range?
All help is welcome and very much appreciated!
Thanks guys.

I'll just post my current system here if theres any chance of upgrading it (which I highly doubt) oh and don't laugh please I've had it for a good while

Some sort of Win Power? 500watt power supply (yeah I've never heard of them either)
Foxconn 6150M2MA-KRS2H motherboard
AMD Athlon 64 X2 Dual Core Processor 5200+ 2.61 GHz
Corsair Value Select VS2GB667D2 2GB of ram
ATI Raedon HD 4650
Hitachi HDT721010SLA360 1TB Hard drive
SAMSUNG HD501LJ 500gb Hard Drive
Monitor is 19" and the resolution is 1440 by 900
Anyway yeah thats that, ideally I was looking to build a new system though and keep my 2 hard drives and monitor (and I also wouldn't need speakers, mouse, keyboard or OS)
I was looking at the i5 and Phenom x2 955 processors and they looked pretty nifty. (Would I notice a big difference with these compared to my current processor?)
Would like a 5770 if possible too.
I would also like to be able to overclock my processor too eventually (I will figure out how to do this one day!)
That's all I really want, I don't really know much about the other hardware like the RAM, CPU cooler and motherboard.
Can you guys recommend me a build (keeping in mind what I said above) that is in my budget range?
All help is welcome and very much appreciated!
Thanks guys.