I have a Dell E521 with the above mentioned 305w PSU. Here are the specs if it helps:
AC Imput 50-60Hz : 100-120V / 9A
200-240V / 4.5A
DC Output +5V / 22A +3.3V / 17A
+12VA / 18A +12VB / 18A
-12V / 1A +5Vfp / 2a
+5V and +3.3v 150w max
+12VA and +12VB 264w max
Anyway it has 22A on the 12v rails.
BTW: I run a XFX 8600GT XXX with no problem
The 8600GTS will also work with the stock psu but you need a molex connection to the psu. Hope this helps.