There is a nice cheap 5.25 -> 1x3.5/4x2.5 adapter from sharkoon. Originally for 1x3.5 + 2x2.5 drives or 4x2.5 drives. You have to clip off a little bit of the lower 2.5" tabs so it fits into the hp case. It will be a 1x3.5 + 1x2.5 adapter then
You need extra:
1x L shaped to straight 50cm sata cable
1x E-sata to sata cable, 50cm
1x 5.25" molex power to dual sata power cable (5-10cm is enough)
With modded HP bios you get 2.5" ssd for system + 5x3.5" drives for raid
Can provide pics if needed.
P.S. you can use 1033 mhz drams on this system ... it works with 800mhz only. Had troubles finding 8gb modules which work, used some of a mac pro 4.1 (1033), these work perfect. Have 2x8GB running in one of my boxes.