I'm not really up to date with all the latest and greatest hardware, however if i was going to be building a new PC for myself I would look at building an Intel Core i5 or i7 machine.
Decide on a good motherboard then build your system around that depending on your budget, I personally go with Asus or Gigabyte for motherboards.
A nice mid range motherboard such as the
Gigabyte GA-P55-USB3 P55 Socket 1156 8 Channel Audio ATX might be a good place to start.
This motherboard supports Core i5, Core i7 processors, memory from PC3-10600 down to PC3-8500, has 8 channel onboard sound, gigabit ethernet, USB3 and Raid.
For the processor something like the
Intel Core i5 650 3.20GHz Socket 1156 4MB would be more than enough.
Hard drive wise i use Seagate or Western Digital, buy according to how much space you want. You can get a nice 1tb drive these days pretty cheaply, you can never have enough space if you ask me.
For memory i don't think you will have any problems as long as you buy known brands, i get Corsair memory myself.
Im not really sure whats the best graphics cards are at the moment, ive not really been in to PC gaming for a while now. I have always gone with the mid range cards at around £150ish and have never had any complaints.
The PALIT GTX 460 SONIC 1024MB looks to get great reviews.
Hopefully someone more in to PC hardware could advise you better, for £600-800 you could build a nice gaming system anyway.
I usually just spend a bit of time reading reviews when im upgrading my PC, you will soon get a good idea of whats good and such.