You can go ahead and make those partitions...
However, you'll end up migrating the data off /home /usr. This isn't such a bad thing, but if you only have the one disk, can be substantially painful.
If you're planning on going LVM right away, I wouldn't bother making the partitions, no.
When you 'convert' the data, you're essentially going to make an LVM volume (/home) and copy the data from the physical partition to the logical (lvm) volume. You can then safely remove the physical partition and mount your logical in it's place.
You can't install Linux using LVM as it's base filetype no.
You *can* convert your root (/) to LVM, but.. there are issues with it. Any upgrades you might do to LVM can potentially hose your root directory.
here is a nice link which has pretty red danger signs on it
http://tldp.org/HOWTO/LVM-HOWTO/upgraderoottolvm.html
My suggestion, and what I typically do myself is to have a small disk dedicated to the OS. That is /, /boot, /dev
These things don't generally change in size. Leave those alone. You can then create volumes for /var, /usr, /home and others (/opt if you go that way) using a second/third/fourth hard drive and LVM.
If you only have the one hard drive I'd heartily recommend installing to / only, parted the drive at a convenient point just above the amount of disk used, create your LVM volume groups using the rest of the space, migrate stuff off the /var /usr /home, parted the original space down to whatever's left (plus a bit), re-assign the now unused space to your LVM and then sit and enjoy the show.
Slick.
