My system is i7 3770 + 8GB RAM + GTX 560 VGA so it should not be the bottleneck.
This is a bad assumption, just because you have a powerful system doesn't mean that your hardware isn't a problem. For instance, I have 2 machine both with Core i7 3970K's (monster chips), 64GB 1600 ram, each with different motherboards. On one I couldn't get my drives to transfer faster then 900MBs (with 8 SSDs drives in a strip set), it turns out that the 2 SATA controllers were sharing one 1xPCIe 2.0 lane. Though the manufacturer had provided 10 SATA ports they didn't actually give it the bandwidth it needed to utilize those ports with SSDs (8 HDDs wouldn't be an issue). The other MB wouldn't accept 3 high bandwidth PCI cards (2xSAS RAID controllers & 40Gbs network card) because they played similar shannanigans with the PCIe bus lanes (splitting lanes). There is a very big difference between what is put on the package (3-4xSLI support is lie) and the compromises they make in design to save $$.
With the price of SSDs falling like a rock you're not doing yourself any great favors by avoiding writing to your SSD. If you wear it out, by the time it is dead, SSDs will be bigger and even less $$. The current line of Vertex 4's are rated for 2,000,000 hours, that would take you 9.5 years of 24/7 usage to failure. The logic that people have towards their SSDs is outdated and comes from the first generation drives which had no proper garbage collection and wear routines. Current generation drives have technology which will extend the dives life considerably (garbage collection, wear leveling, additional memory which is used to replace bad cells). So stop babying your SSD it's a Ferrari, don't drive it like it's your momma's 30 year old station wagon.
You're 5400 RPM drive is the bottleneck (anything below 7200RPM is a waste IMHO) and really is best used for infrequently accessed or slow data (Mp3s, pics, videos,etc). You can mask it's slowness with caching but you'll still experience it's it fairly often just by the nature of how caching works (all caching). My advice is clear off any garbage you don't need (especially any junkware installed by Dell, HP, Lenovo, etc) of your SSD, install nonessential slow programs to your 5400 drive (say your CDburning software, Skype, MAME, Virtual DJ and run fancy cache to smooth out the experience) then put the stuff that has to run fast on the SSD, (Web Browser, MS Office, current fav game). Just be mindful to keep the drive less then 3/4 full as it needs space to move things around during maintenance.
To answer your question about the C drive, it wouldn't do you any good. People think that drive access is the bottleneck for slow machines and sometimes it is but most of the time it isn't. Software and especially H/W drivers are your biggest culprits and mostly it comes down to data processing and timeouts. On my fancycache setup I have 26 active virtual machines (26 C drives, 12 data drives) and when I monitor drive transfer rates and ques, you'd be shocked how little your C drive is actually accessed. Most of the time I'm seeing about 10-25MB R/W transfer rates (on all 26 machines at once) on a drive that can do 3,500MBs. So caching the C drive while possible (I'm doing it with virtualization) doesn't really make that much of a difference.