rack mount are for servers only, i have never seen any desktop/workstation in rackmount. there is no way you can setup your current hardware in rackmount also, as rack mount comes with a giant heat spreader on the CPU and fans are located at the very end of the system, that draws in the air from front and pushes out at the back.
as far as slim cases are concerned, you may will be able to put in your motherboard, BUT, the graphics card/sound card has to be low profile to fit in the casing (which are inlimited number and are of limited capabilities).
only thing u'll be left with is Shuttle/Asus/MSI barebone systems.
Intel Core2Quad 2.66Ghz 12MB L2-Cache
4096MB DDR Kingston Hyper-X
XFX 9600GT AlphaDog
Seagate 2 X 320GB SATA2
LG L246WH Flatron 24"