Haro Rider 09: Let's start off by asking what kind (specific) of bike do you have now? Some bikes are definitely worth upgrading the parts on, but if you don't have good rims, then what is the point of spending 5 hours rebuilding nice hubs into the crappy rims?

If you don't have 3-piece cranks, I don't think you will be finding any sealed bottom brackets to put on your bike, so you need 3-piece cranks.

A sealed headset it a toughie. Headsets take a lot of up/down force against the bearings which is not what cartridge bearings were designed to handle. So you may be better off with a loose ball bearing headset. But, some people love their sealed bearing headsets. As I said, it is a tough question.

If your bike really needs everything upgraded, then it may be time to sell it on eBay and pick up a $400.00-$500.00 bike. The more expensive bike will come with decent sealed components, full chromoly frame/fork and better components. You may still want to upgrade stuff later on, but then you won't be in the situation where EVERYTHING needs to be upgraded, but just small stuff. Like your headset.