Major PC upgrade (Virtually new build), I have a few questions, never built a new PC.?

I purchased a Dell XPS 8100 last year, which was excellent with the exception of a low powered 350W power supply and an average GTS 240 GPU. So I decided to purchase a CORSAIR 750W Power supply & a Nvidia 570 GTX Graphics card. But when I went to replace the power supply I found that the old power supply could not be removed and with the case being so small my new power supply wouldn't fit anyway. The other problem I found was that the Dell motherboard had a unique 4 pin power input at the top of the board additional to the 24 pin connection, when the 4 pin connector was unplugged the PC wouldn't power up, so it's not some kind of surge protection. So now, annoyed at Dell's attempt to provide awkward hardware I've decided to purchase a new ASUS P7P5SD-E PRO motherboard, which has SLI, crossfire, USB 3.0 etc. I'm also getting a Nzxt Phantom PC Case and an OCZ Technology 120Gb Vertex 2 SATA II SSD. Additional to the parts already mentioned I will take from my old PC my 8 Gb DDR 3 RAM, 860 i7 (I'm assuming the heat-sink/fan for the CPU will fit my new board), 2Tb hard drive, Network card & a Blu-ray optical drive. So my first question would be, when I plug everything in, should it boot up if connected to the hard drive with the OS? Will I have any problems with BIOS having removed the Dell motherboard & will it boot with the new motherboard? Will I have to purchase a new OS? My intention is to transfer the operating system onto the SSD and is it possible to copy the files over from the 2TB hard drive? I'm hoping to build it and set it up myself, but I'm hoping whether someone with more experience would let me know if I would be better of booking it into PC world?