The upgrade from 5.6 to 6.0 should run OK, but watch out for any contribs you have installed. Some need to be upgraded for v6.0 eg arkeia, backup2ws, LAT, & I think also the user manager/panel contrib. There have been some posts about which contribs work OK with v6.0 but most authors have now updated their packages. You should install the new ones.
Also watch out for lock up problems if you have anti virus scanning and particularly spamassassin enabled. Low end machines with small amout of RAM that worked quite OK with v5.6 will probably be a problem under load conditions. Ideally to run v6.0 with various contribs installed and running you want a minimum processor of 500MHz (or better) and 512Mb RAM. There have been numerous posts about problems people have had with low end machines locking up and I had the problem also.