they get very hot in their stock configuration, if you want to run the high end stuff and be reasonably confident that it won't go poof then be prepared to do some modding.
i have a SB51G and had serious concerns, primarily about how hot the PS gets(ahcme brand in the SB51G) and an AGP vid card. i am running a 1.8A @ 2.6 (had to remove a pin - the bios recognizes 'A's and sets the available fsb from 100-132), a Ti4600, and a WD 80gb JB drive.
i started with the heatpipe 'radiator' - replaced the stock 80mm sunon w/ a panaflo L1A pulling air into the case - as the stock setup blows already hot case air through the radiator (found pulling air in to be more effective back in the t-bird water cooling days), then added another L1A to the heatpipe heatsink (zip tied on).
next the PS - i ran the system with the case open, and the side of the PS removed and i was amazed at how scortchingly hot the aluminium sinks inside the PS got - ceratinly hot enough to melt any wire insulation that might touch them and hot enough to give fingers a severe burn instantly. to deal with it, i pulled the tiny and loud little sunon 40mm fan out, cut the finger guard out of the PS, and put it back in the case without replacing the side of the PS - so now the air that is drawn through the PS isn't restricted to coming in via the tiny vents at it's rear (the PS is so packed that there is almost no way airflow could be effective). i took an old 60mm fan and cut the fan out of it and taped its cage onto a 60mm SUNON 21cfm fan and plugged it into header 1 on the mobo so it's speed can be software controlled. the assembly was then attached externally pulling air out of the PS/case.
finally using a hole saw i cut a 2" hole over the Ti4600 fan, and attached a finger guard - the aluminium case is thin enough that any reasonbly sharp hole saw makes short work of it.
now not only does it run significantly cooler, but a good deal quieter.
a picture