I live in the area but not sure if I'm about at the time (moving jobs to a local College and all that)
Get an OS map as loads of trails over the Preseli hills (mountains is pushing it a bit, although my wife insists they are; she's local from birth...)
If you're looking at the hills, start and finish at Rosebush, as a great pub and tea shop there (seperate business') and big car park. They have also just opened a walking/cycling/horse riding area in what was (and will be again) forsetry area close to the car park; follow the unmade road through the old quarry.
Careful riding the coast path as it's walkers only...
There is a growing website: http://www.thewayoutwest.co.uk/category/mountain_biking
that details some good riding in the county by locals.
Although Pembs is mainly frequented by roadies, mountain biking is VERY slowly growing.
Give me a shout when you're here and I'll see if me, my PACE and my mate, on his 'modern' are able to meet up; even if only for an evening ride/natter/beer. PM me and I'll send you my mobile number
Been MTB racing since the 80s but now just riding for the sheer joy of it...
Current rides: Yeti AS-R & Graham Tomlinson 531c Road Bike & CBR Gatecrasher (for my wife)
Building: X-Rated Snare BMX