You can't mix RAM sticks. I'm not sure why it's giving you that option. You'd be fine with just the 2x4gb.
That's an outdated graphics card. What are your options? A 6770 would be nice and ~$100.
I stick by my above comment.
The 2600k is a great CPU .... whether you need it, will depend upon whether the apps you are using are multi-threaded. If the answer is yes, the 2600k or 2700k would be on my buy list. If not, then there's a good argument to drop down to the 2500k.....you have to decide whether the extra $100 is worth the 0.1 GHz speed bump. Each of your options however has no "K", I would not buy the non K versions.
I'd suggest limiting ya RAM modules to 2 at this point......though I don't have much experience in this regard w/ SB, with the I7-9xx series, you oft would have ya CPU OC's gimped a bit when filling that 2nd set of RAM slots (unless ya dropped RAM speed amd / or timings) .... and always best not to mix RAM sizes. And I wouldn't get less than DDR3-1600 ..... and 8 GB is sufficient for the apps you stated..... RAID won't help unless ya spreadsheets are H U G E.....an SSD will reduce ya Windows boot time from like 21 w/ a good HD to 16 seconds w/ the SSD.
For optical I'd suggest a combo BR Player DVD-RW....recording media for BR is too expensive.
The 560 Ti is the best card there.... if you into gaming ..... or if any of ya apps use CUDA.
Here's a recent build I did for a web design guy who also had video editing / gaming interests .... he needed all the peripherals, including monitor KB, Mouse , OS .... Total was $ 2,185 or about 1,4000 GBP w/o those price would be $1,600 or 1,030 GBP
I'll be using applications such as Wordpress, Dreamweaver, Photoshop and Apache, PHP & MySQL. Please could someone advise which components matter the most?
I won't be using my computer for gaming.
Those programs are not demanding at all. They wouldn't tax any decent rig. Except maybe Photoshop sometimes, where an SSD and 16G of RAM would make things go in the blink of an eye. You have to decide whether it's worth several hundred dollars to avoid a few seconds' wait now and then.