OK, at the outset I'll 'fess up and acknowledge that what one needs and what one wants are two separate entities and I wanted to build a new PC.
I documented my last build here
some four years ago and my choice of an Intel Core i7-920 wasn't exactly bleeding edge but it wasn't too shabby either. The new machine hosts a Haswell i7-4770K set to a conservative overclock of 4.4GHz, memory is up from 6GB to 32GB (1866MHz) and I've sprung for a GTX Titan graphics card. I've run a few benchmarks and pretty much as expected I'm seeing about twice the speed on CPU intensive tasks and up to three times the speed on GPU intensive tasks. I could probably have achieved most of the latter improvement by simply upgrading the graphics card and if I had had a Sandy Bridge CPU/motherboard then that graphics card upgrade might have been all I needed...
So far so ho-hum but for this build I decided to ditch the hard drives and go entirely solid state, with a single 400GB SSD connected to the motherboard's SATA controller as my C: drive and four more of the same drives connected to an LSI MegaRAID controller and configured as a RAID 0 D: drive. At today's prices those enterprise class drives (rated for 5 complete overwrites every day for five years - the 200GB version is reviewed here
) are crazily expensive but in a few years prices will almost certainly have dropped to mainstream levels. By the way, while RAID 0 offers no redundancy I took the view, particularly bearing in mind my choice of Intel DC S3700 drives, that I'd be very unlucky to see a drive failure ever and, of course, I always back up my data!
For comparison, on my old machine with four Samsung Spinpoint F1 drives hanging off an Adaptec controller and configured as RAID 10 I could, from the D: drive, load and render a many layered 258MB Photoshop PSD file in 14.6 seconds. On the new machine the same operation, also from the D: drive, takes 4.7 seconds.
Taking a wild guess I'd offer that if you already have a Sandy Bridge machine then you may see a speed improvement if you upgrade an older graphics card. If you have the budget to ditch your HDDs and go for SSDs instead then that might offer the most noticeable benefit if you routinely access large files. For me, taking as I do the view that CPU performance is plateauing, this new machine should hopefully still be my main workstation in ten years time. That way I can, sort of, justify the cost.