Overclock my pre-built Dell (just bought)

slightly older model Dell Optiplex 740 Enhanced (AMD)
bought it from work to play wit hand would like to know in advance about any overclocking.
BIOS seems locked, any tweaking at all?
I have a custom build and this is my first pre-built (excluding laptops) that I have ever had at home.
what can I do with it?
specs:
athlon x2 7850 black edition
4GB(2x2) Corsair XMS DDR2 800MHz
Dell OEM GTS 240
2 x WD VelociRaptor 300GB 10K - RAID0
3.5 media card reader
430-watt Thermaltake TRU-430
80mm case fan