Running SH5 at full whack will need something along the lines of the following:
8 Gig decent RAM (1333 mhz min, ideally 1600)
Quad core CPU & 64bit OS - the Core i5 2500k is the go-to CPU for the current gaming generation and should last quite a bit longer too. Also quite reasonable now the new Intel CPUs (which have only a very small boost over the last gen) are out.
A GPU with at LEAST 1 gig VRAM, preferably higher. I run a 1.3 gig Nvidia GTX 570 and it's a monster, and in my experience Nvidia are less hassle than ATI, and have better shadow rendering. The newer Nvidia cards also have access to a new AntiAliasing mode - FXAA - which should give a performance boost over other methods, and new Adaptive V-Sync. Anything from the 5xx series up should do you fine.
Anything else is fluff. Bear in mind also that your monitor resolution will affect performance. I run a setup very similar to the above (slightly older CPU) @ 1900 x 1200 and it copes just fine.
Edit: Webster's spot on about the PSU and case-fans etc as well. Don't waste money on cheap coolers, buy a proper one! Prolimatech do some awesome ones, I use an old Megahalem but their newer range is excellent too.