Need help before purchasing new OS

Im looking to purchase WIndows Vista for my new computer

Specs:

-MSI K9A2 Platinum, AMD 790FX, AM2+, PCI-E 2.0 (x16), DDR2 533/667/800/1066, SATA II, SATA RAID, ATX
-AMD AM2 Athlon 64 6000+ AM2 , Windsor Core, Dual Core 3.0GHz, 1MB x 2 Cache
-Sapphire HD4870 512Mb PCI-Express graphics card
-Western Digital Caviar 500GB SATA 3GB/S Internal Hard drive
-2GB (2x1GB) Corsair TwinX XMS2, DDR2 PC2-6400 (800), 240 Pins, Non-ECC Unbuffered, CAS 4-4-4-12
-Coolermaster CM-690 V2 Dominator Black Midi Tower Case with Mesh Top Panel w/o PSU
-750W TX Corsair PSU, single 12V rail, energy efficient, quiet & cool, fully compatible
-Samsung SH-S223F/BEBE 22x DVD±R, 12x DVD±R, DVD+RW x8/-RW x6, SATA, Black, OEM

^^Not been bought yet... could upgrade to 4GB of ram easily.

Right to business, i have found two choices, but have a couple of questions i would like answered.
http://www.amazon.co.uk/Windows-Vi [...] 522&sr=1-4
http://www.amazon.co.uk/gp/product [...] 26&seller=

1. What does DSP/OEI actually mean? I understand what OEM is. Are they the same thing?
2. Are these the full operating systems with SP1? or just SP1 to upgrade an existing OS?
3. Should i get Ultimate or Home Premium, i plan on using it for gaming/photoshop/web browsing.

Cheers in advance, hope you can help me.