Hi 1speed,
Only live about Afan Argoed, where you have the man made trails of Penhydd and the Wal. Aslo up the Valley at Glyncorrwg you have White's Level, July and Skyline Trails.
In and around Machynlleth is really good. The people are very chilled and laid back. I know the guys really wellwho run the Holey Trail bike shop there, and get up there at least once every two months. Done the Dyfi Enduro 6 years in a row, and singlespeed'ed it this year (40 miles with 8000 ft of climbing all in the Dyfi Forest).
Elan Valley is awesome too.
Also you have the man made climachx trail, and further to the south towards Aberystwyth Nant y Arian.
The Beacons are awesome, as well as Fforest Fawr and Sarn Helen to the west, and the Black Mountains to the east. All awesome riding.
Also up Snowdonia is obviously awesome too.
some pics for your enjoyment:
http://kluster.frankencrank.com/images/whites%20march%2006%20opt/
http://kluster.frankencrank.com/images/the%20black%20mountain/
http://kluster.frankencrank.com/images/sarn%20snow%20web/
http://kluster.frankencrank.com/images/90%20offroad/

This is one of my fav rides:
http://kluster.frankencrank.com/images/pont%20scethin%20and%20climachx%20web%20pics/

Out tomorrow for a ride around Skyline, got to get some training in for the Merida in Builth Well on July the 8th.
http://kluster.frankencrank.com/images/skyline/
All I have to do now is get out and take some more pics of Stones.

Some bike pics:
http://kluster.frankencrank.com/images/Epic%20Carbon%20Modify%20Web/
http://kluster.frankencrank.com/images/roadbike/cannondale_01.jpg