Where to start with upgrades?

[therealpigdog]

I should be getting a bit of spare cash next month, and having just upgraded to Sky HD, I'd like to get the most out of my system and hopefully upgrade my overall media centre experience. I've posted here rather than computers as it's really the TV and Hifi experience that I want to improve, although I appreciate there is some crossover in terms of hardware.

Current system:

• 42" LG Plasma TV 720p - very happy with this and can't see any reason to upgrade (My eyes aren't perfect and I sit too far away from the screen for 1080 to make any discernible difference - c16ft).
• Sky+ HD - again, perfectly happy with this so far.
• Playstation 3 - Used as a bluray and media centre, with very occasional gaming (once a month if that).
• Old PC connected to wireless network and used as a media server - 500GB hard drive (ide) full of music and ripped DVDs. No HD content yet, but looking into it.
• Nothing else connected to the network - one freeview tv in another room that is hardly ever used, and various iPod docks and DAB radios that are used all the time.

I'm thinking that I first need to improve the sound, as currently using the TV sound only, so have looked at an Onkyo HT-X22HDX 2.1 surround sound system that I think would fulfil my needs. I can't use wired rear speakers (without paying for them to be channelled into walls), so think 2.1 would be the best compromise. Cost is about £230 for the unit.

Rather than use a NAS drive, I can simply upgrade the PS3 hard drive to 1TB and then simply transfer all my music and films onto it directly. This would mean that I don't need a separate server, and so can remove the old PC from the equation. Cost for this is approx £70. Would a NAS drive be better? Is there any benefit in upgrading my old PC that is used as a server (it is a £20 pentium, so very slow).

I use iTunes to sort my music and film library, and really can't be bothered changing unless I absolutely have to as we have various iPods and iPhones that we like. Everything is stored on my laptop and then backed up to several big USB drives, so appreciate that it would be duplicated on the PS3 hard drive and wouldn't automatically sync, but I can live with that (unless there is a better way). I'd rather avoid having the laptop turned on to use digital content if possible.

Is there any benefit in using homeplugs to connect both the sky box and PS3 to my network (currently wireless)? Probably would be at the moment, but not if I went for the bigger PS3 hard drive rather than a NAS drive.

I like the idea of sonos etc to play music around the house, but don't think it's worth it just yet and understand that they aren't great with iTunes libraries - I'm happy just keeping my iPod classic plugged into the one in the kitchen and updating through iTunes every now and then.

I don't want to spend the earth, but want best performance upgrade for my buck - happy to do it in stages as finances dictate.

What would your suggestions be?