I know the distros are targeted at users with older hardware,
Actually there are Gnu/Linux distros aimed at backward compat to older hardware: example, Absolute Linux based on Slackware. But most distros are aimed to cater user needs --not necessarily for backward hardware. See for example Opensuse, Ubuntu, Mint, Redhat, Fedora etc. these are not aimed to older hardware but to provide cutting edge performance for both users with latest hardware and the bit less current; yet, it is impossible for those distros to run on a 466hz Celeron on a 16Mb RAM. To say that (ALL) Distros ARE targeted at users with older hardware is a complete mistake.
In comparison rather, <as a matter of joke> saying that M$Windows v.($) is universally poor OS is a truism.
but it seems to me that the less resources used by the OS would result in more resources being available in userspace amirite?
The utilization of user-space is not anchored on the size of RAM reserved by the kernel. This condition is a case to case basis depending on the software being called to require resources. Example, GWview (a Gnome image viewer) regardless of conditions raised by the kernel or the size of userspace normally commands 10MB from the point it starts.
If you have that big RAM resources and Quad Processors that means that you have 4 highways to process tasks and ample working space. You can run MSWindows v.($) fast enough on it: BUT with Slackware (Ubuntu, Debian, Fedora, Suse, etc.) FASTER.
Hope that helps.
Congrats for a cutting edge hardware.