The old engines use two threads,all newer engines use at least 4 threads:
1.)CryEngine 3 - 4 threads are used
2.)idTech 5 - even 8 threads can be used
3.)Frostbite 1.5 and 2 - 4 threads are used
4.)Later UE3 games - 4 threads are used(check the latest Deus Ex game)
5.)RED engine - 4 threads are used
UE4 will probably use at least 4 threads and is being demoed this year.
Older engines will use upto two threads but are now at their swansong.
So at least three to four threads are required. So a core i5 if you don't want to change the CPU for a few years,or a Core i3 for the time-being are what you should be considering. If FRAPs is not using the CPU that much it must be saving uncompressed screen grabs to your hard drive. If it were compressing them on the go that would use your CPU.
The 4 RAM slot capable B75 and H77 motherboards can be had for around £55 to £60 from Ebuyer but are mATX. ATX motherboards are closer to around £70 and you would want the H67 and Z68 ones,so you can use Quick Sync. The newer Z77 motherboards are nearer to £80. P67 ones with 4 RAM slots can be had for between £60 to £70 but you cannot use Quick Sync.
I had a look around for the cheapest Core i5.
You can get this Core i5 2320 for around £130 delivered:
http://www.novatech.co.uk/products/c...623i52320.html
TopCashback do 3.03% cashback on Novatech purchases,so that should push down the price to around £126.
Using Scan free postage you can get the following parts.
The total comes to around £660. The ZS series PSUs are not modular so cable routing will be a bit more different.
You might be able to get the HD7850 2GB a tad cheaper elsewhere,but the single fan cards are supposed to run a tad hot. RAM might be cheap delivered elsewhere but as long as it is decent 1.5V rated 1333MHZ or 1600MHZ RAM it will be fine.
Only 64 bit Windows will recognise all the RAM. However,8GB should be plenty and save you around £40.
Use the 160GB SATA drives to record you videos.