An upgrade to artist or the full version of cubase is never a bad idea. The included vsti's are usefull but maybe not what you are looking for. But since you do record vocals, having (a lot) more options on that aspect is allways an upgrade for your rig.
If you want real sounding instruments then you could take a look into the many available packages on the market. They are never included in any DAW standard package. They contain most of the times a decent string, a decent flute, ... and so on, but that is not to be compared with what the sample bank builders are offering.
High quality natural instruments come from brands like East West, Vienna, cinesamples, Project SAM, Steinberg Halion Symphonic, Garritan and so on... They vary a lot in price and in functionality. Also: most of them are not cheap. Try before buy is the best thing you can do here and feal if a) the sound is what you are looking for, b) the workflow is what you want. Another thing you can do is stepping in one of the combined deals a lot of those brands offer. When getting bargains up to 75% in most cases there is no reason to regret any of those buys. You wil get multiple gigabytes of samples. Most of them are top quality.
I would not opt for the "complete" package of NI since the normal complete package is more a bundle of very old sample banks and vsti's. But that said, the combination offered is a bargain overall deal but not specificly for natural sounding instruments. Different story with their extended complete package. That one is 1000 euro's but then you get a really hefty bunch of vsti's.
But that said, the same with the CCC2 deals of East West. This is top class stuff. Same with the Vienna bundles.
So there is no single path to go, but neither one of the above mentioned brands will fail to deliver what they promise.
My personal findings on those package:
- East West is more Pop/Hollywood oriented. But very high grade. Their guitares are the best i've heard.
- Vienna is more classic music oriented. The wealthy amount of variations in playstyle is a big selling point and awesome.
- Garritan: very affordable and excellent sound, but needs some tweaking
- Halion Symphonic: a bit a mixed bag, very affordable (99 euro) and outstanding on some points but needs some tweaking too.
Another option is to go for a workstation. The results are comparable with what you have with your XP though. They offer a very broad scale of sounds, but they are allways less expressive/realistic then the sample banks mentioned above. The advantage is they cost a lot less then the specific sample banks.
Probably other users will want to add to this comment...