Looks like my plans for this (spurred on by a couple of posts by Cat) are taking off! It's basically down to a matter of how choosing which PSU/GPU combo to go for now.
Intel Pentium Dual Core E5200
Zotac 9300-ITX WiFi Motherboard
2GB PC2-6400 RAM (downgrade from 4GB, but i'll replace the 2 1GB DIMMs eventually)
250GB Samsung Spinpoint (my current HDD)
Nexus Psile Case w/ slot loading DVD drive (Found a bargain on this case! Comes in £50 more than the SG05, but with the DVD drive and minus the PSU)
120W PicoPSU or 200W PW-200
Add in a 4670, 4750 or (ideally) 4770 and i'm good to go!
Total cost isn't too bad either, i end up something like £150 out of pocket once i've sold my existing kit. Not a huge upgrade in performance, but more than enough for my needs. If i go for the PW-200 and a 4670/4750 then i could easily get away with quite a substantial overclock on that CPU. With the 4770 i might be pushing my luck a little.
Will do a build log once i start. Might be another couple of weeks yet, need to decide whether or not to wait til pay day.