You can use the standard windows app (included in the unwhitelisted bios also available on myhpmini). Be very careful with the settings, some people have bricked their mini's. The settings are a little more technical than your usual bios, and lack of ability to reset the cmos = bricked if wrong settings.
So far I have had good results (using a 1066 Corsair 2gb stick), turning off all spread spectrum settings, and in performance using linked 3:2 ratio, 512 + 224. This gives approx 2.2ghz cpu and 1100 mhz ddr3 ram.
Most of the time if you get a blank screen extracting the added ram will let you back in the bios.
Everest memory scores with Corsair 2gb stick
Stock speeds
3951 mb/s read
2839 mb/s write
2480 mb/s copy
104.4 ns latency
OC with OCtuner (s/w) up to 160mhz fsb (about 1.9ghz if I recall)
4605 mb/s read
3403 mb/s write
3012 mb/s copy
77.9 ns latency
OC with Bios 2.2ghz 1100 memory
4888 mb/s read
3634 mb/s write
3283 mb/s copy
83.1ns latency
Windows Experience Index
Stock speeds (after adding 2gb ram in brackets)
Processor - 2.3
Memory - 3.9 - (4.5)
Graphics Aero - 4.2 - (4.5)
Gaming graphics - 5.3
Primary hard disk - 5.6
With OC Tuner (s/w overclock)
Processor - 2.6
Memory - 4.7
Graphics Aero - 4.7
Gaming graphics - 5.4
Primary hard disk - 5.7
With Bios OC
Processor - 3.1
Memory - 4.9
Graphics Aero - 4.9
Gaming graphics - 5.4
Primary hard disk - 5.7