Here's my take on onboard GPUs
They tend to be lower powered, but most modern motherboards seem to at least have usable ones - so you would need to check the specific chipset. Another issue is they often use shared memory, stolen from your main allocation, which does seem to upset some programs. However, if you are building a desktop, as long as you make sure the motherboard has a slot for adding a graphics card later, you are covered, really. Studio uses the GPU for many tasks, not just 3D rendering, so you should see the benefit of a good GPU and dedicated memory.
Sound - I've not had a separate sound card on my last two computers and I don't think it's a requirement at all
If you get HD later, you can slot a new graphics card in - although what works for SD should work (slower) for HD. Won't worry about sound....
S12 still treats DV video in the same way as S9 without additions.
Have you tried the S12 trial on your current hardware? It should run quite well 
If you are specing up a new computer, two things I'd suggest you check with your new motherboard.
One is that if has sufficient expansion slots - mine only has 2xPCI, 1xPCI-ex16 and 1xPCI-ex1 and I've run out of slots - I've even found a use for the PCI-Ex1 - a wireless network card - so if I wanted to add a sound card I'd have to remove my USB/Firewire expansion card.
Two is the number of internal HDDs you are likely to want to add. Some new Mobos skimp or omit completely PATA HDD connections, so you won't be able to salvage old drives. Others also might only have 2 SATA ports. My last upgrade relied on using two HDDs and a PATA DVD burner from the last computer, so I got a board with 4xpata and 2xsata, and they are now all in use. (1.5 terrabytes not counting USB connections).
Anyone else?