Price range?
I am wondering if you could not get the Megaworks 5.1 at reasonable price now (afterall, the Gigaworks are replacing it. For most intent and purposes, the MW -are- pretty close to the Gigaworks though.
Between the Z-5500 and the Promedia... Toughie. The Z-5500 is pretty new, and I've not heard it. I've heard its predecessor, the Z680, and well, I would have to agree with the folks who find it bassheavy (this applies to its predecessor too - which is why I don't think they've changed their formula). At the time I was comparing the Z-680 and the Megaworks, prefered the later. Thats why I went with the Gigaworks.
The Promedia, well... Actually where do you see them sold in the UK? I was seriously considering the Klipsh Ultra over the Gigaworks, but I gave up trying to find Klipsh speakers in the UK. The Promedia 5.1 have been around for a good while and it has been a -very- long time since I've had an encounter. I can't comment much on it now, but I gave it the thumb up back then, except that getting it over to the UK just wasn't going to happen.
My final verdict: Gigaworks if the budget allows, or get the Megaworks
If the Promedia is around, then you might check if the place also sell the Ultras (my guess though, is that they'll cost a fortune).