It all depends on what you want to do (and which platform)
In the x64 area, Mint only comes in Gnome and you install KDE (it looks like you want KDE from your post)
Also, in the x64 area, and (at least) AMD Phenomx4, I needed the 2.6.27 kernel or greater due to (I think) cpu frequencies although I think NVIDIA benefited there, too.
If you have a dual core, you should be fine.
What I did on my Phenom x4 pc, was install Mint 7, change jaunty to karmic in /etc/apt/sources.list and did a
apt-get update && apt-get dist-upgrade.
I now have Mint 7 built on top of Karmic. Granted, this is Gnome, but I can install KDE. Also the KDE Mint is 32 bit only AFAIK.
I'd expect Mint 8 to have a similar upgrade path (as I look in to the past Mint releases) for it Mint additions to Karmic.
Hope this helps with your decision.