As this is obviously your first Windows Mobile phone, so you need to realise that what makes a WM5/6 device so great is the software you install. Everything that comes with the device as standard is very basic and watered down. But you have definatly made the right choice is it's streaming you want, Symbain smart phones (Nokia/SE) are utterly pathetic when it comes to stuff like this. Windows Mobile offers a whole other universe of joy
Although there is plenty of other software out there, there are two ways of streaming i personally use at home which are fantastic...
The general networking method (for accessing all files on my PC); Get
Resco File Explorer. It's not free, but it's absolutely must have software for windows mobile. It should be a law to have it, it's the one first program most people who want to make the most from there device will install. As well as many other great features, it's a very good networking tool. It's allows you to 'Map' network shares (folder on you PC you have set for sharing). You can then view/modify/run any file from just about any program on your XDA (like opening a word document thats on your PC). The folder on the PC will just appear to be another folder on you phone. I personally recommend TCPMP for general video playing, it's about the best and it's free.
The slightly slicker method (for media files only); Download
www.TVersity.com and install it on you PC. It's a free UPNP server (Universal Plug 'N Play - the same method the XBOX360 etc uses to stream from a PC). Then get
Conduits Pocket Player which as well as being the best music player (i still think TCPMP is the best video player) it will connect to a UPNP server. you might also need the UPNP plugin (available on the Conduits site site). Then when running PP, just hit browse-network ad you can search all you music but artist, album name etc. It you could use PP with a general networking with Resco, but UPNP gives you a nice and quicker interface to search with etc, rather then having to go through loads of folders and files to find what you want.
Assuming you have no problem connecting the XDA to you network, you can enjoy all your music and video around the home/garden. Get an adaptor and phono lead off eBay for a couple of quid and you can run it though your hifi for a cheap mans Squeezebox
As for streaming YouTube and other video directly from the net, yes you can. I don't do it myself, but there are plenty of methods. If you want to stream to PC content to your device while you are away from home, there is stuff like Orb etc. HOWEVER; unless you find a hotspot, A) the XDA Mini S (HTC Wizard) is not 3G and GPRS is not fast enough, and B) O2 data charges are astronomical. They are so expensive compared to T-Mobile etc that you have to wonder if they actually want people to use it.