First off, please excuse my ignorance..

OK, so I have a Bianchi Advantage that I really like as my daily commuter, seriously fredicated but it suits my needs well. Well, it is getting to the point that I want to upgrade my SRAM X.4 rear derailleur and options seem rather limited.

Would upgrading to 10 speeds be simply a matter of replacing the cassette, rear der and shifter? Will a 10 speed derailleur fit on the existing hub that is now holding an 8 speed cassette?

Any constructive advice is greatly appreciated!

You would also need at least the chain (narrower) , crankset, and by the time you buy all this it would be cheaper to buy a new bike. If you CL shopped, parts meet shopped etc. you might do it cheaper but things may or may network well together and you end up buying them twice. For what its worth going to a ten speed will give you the same lowest gear and the same highest gear with just more unusable gears in between. Roger

Just a slight correction: There would be no need to upgrade the crankset when moving to a 10 speed cassette. "10 speed" specific cranksets are slightly modified to shift better with the narrower chain but that doesn't mean you must use one. A 10 speed cassette will fit just fine on an 8/9 speed freehub. Furthermore, most 8/9 speed rear deraillers (Shimano, not sure about SRAM) will work fine for a 10 speed system. The indexing is all done by the shifter.

rhenning is correct that merely moving to more cogs won't give you any more range, just smaller steps (useful to some but not all cyclists). As the cassette range widens, smaller steps become more important to me. I'm ok at 8 speeds for a 12-25 but even a 9 speed 11-32 doesn't agree with my pedaling style on the road (offroad I'm more tolerant of the gaps).

Thanks for the advice. The driver here is not to get more gears, but to upgrade from the old worn out X.4 derailleur to another that will shift better. There just doesn't seem to be a lot of options without buying used equipment that might leave me right where I already am. The der that is on there works, just not really well. I guess it works as well as I would expect a $20 der to work.

I already have way too much money sunk in to this bike, so some more isn't going to make a huge difference.. lol ..

Even the most basic derailleurs shoudl shift perfectly when new - if it is adjusted properly. Generally speaking, higher end derailleurs can resist wear over time and be easier to keep adjusted.
Whatever you replace your X4 with will be perfect if you adjust it properly. Don't bother with all the 'upgrade' hype - the limiting factor is not the price of the components but your skill in adjusting it.

Edit:

This is what is called 'throwing good money after bad.' Spend what you need to to keep the bike working or make it more suitable for you, but don't start any major upgrade or conversion projects as this is just a waste. If you want a lightweight racey high-end bike, buy a high end bike.

In what way does your current derailleur not work well ? Maybe you have rusted cables, or a kink in the housing, or the hanger is a bit bent , or the cage is bent, or ... or .....