I would see how it performs for your needs before putting a lot of money into performance upgrades, it may be enough for you. I know back when my rig was new and still stock it was fast enough for my needs,

Another popular upgrade is to do an engine swap when the 3.0 gives up, is to do a 3.4 swap, or the Vortec 4.3, both run about $4500, some have done it cheaper, it depends on how many parts you buy or are able to make yourself. I considered the 3.4 swap 2 yrs ago, but I was unable to rebuild the top end and add headers for about the same cost as the wiring conversion harness at the time.