Just for checking to see what hardware might be a problem, you might try the kde classic live version.
I doubt you're both an Openbox and Kde fan, but this can check what works out-of-the box in an easy environment, without installing.
There will be issues with Nvidia; most answers are already in this forum. Please post back if you decide to go ahead and you encounter these problems.
As for which to install, choose Standard. I use and love Light, but there's more community familiarity with Standard, so it's better tested, etc. I've a single core 32-bit via here, and the Kde live version is even bearable for speed; you definitely have the horses to run Standard happily.
Latest version of Openbox currently in the repos is 220.127.116.11. Subject to change, of course. Nearly all Slackware 12.1 pacakges will work, so will many Slack 13 packages. Repo also has lxde-common at 0.3.2.1 and associated packages.
Others will respond to your post also. Are you planning to start VL in a separate partition at first, or to otherwise keep your current distro available while checking out Vector? Knowing this might affect opinions given.
Have fun, hth