1. Largely just quality, but also the memory timings vary (CAS latency, etc) - lower the better. Wouldn't say its worth going for any of the insanely priced kits, but GeIL value is nice stuff and fairly cheap so its worth getting ahead of the OCUK Ownbrand, I'd say.
2. Its not how much memory it has, its how it uses it that counts - no point in storing loads of textures if the moment it tries to render anything it dies. Get the X1800XT 256mb that's on special at the minute, they're great and its insanely cheap right now.
3. Quality over quantity. SLI motherboard doesn't make any difference to the power drain unless you're actually running SLI. Really, a good quality 500w or so would be a nice idea, but above that starts to become overkill if you're talking top brand stuff and not too much in the way of extras and no SLI.
4. Its a quality item and if you want to overclock your computer its a great buy. If you don't its probably overkill and keeping the change is a better idea.
5. Extra features. I don't know exactly what they are, or if they'd be worth waiting for though.
6. You answered your own question - 2 is better than 1

Of course, money wise its not really worth getting two cards, you might as just get one slightly pricer one and pocket the change.
Anyway, make good use of this weeks special offers, they include a rather large amount of what you need with a rather good amount off:
AMD Athlon 64 X2 Dual Core 3800+ (Socket 939) - Retail (ADA3800BVBOX) (CP-134-AM)
Price: £175.95 (£206.74)
Asus A8R-MVP Crossfire (Socket 939) PCI-Express Motherboard (MB-129-AS)
Price: £63.95 (£75.14 )
GeIL 2GB (2x1GB) PC3200 Value Dual Channel Kit CAS3 (GE2GB3200BDC) (MY-046-GL)
Price: £89.95 (£105.69 )
Connect3D ATI Radeon X1800 XT 256MB GDDR3 AVIVO TV-Out/Dual DVI (PCI-Express) - Retail (GX-044-CO)
Price: £169.95 (£199.69 Including VAT at 17.5%)
Enermax Noisetaker 485W EG495AX-VE(G) SFMA ATX2.0 PSU (CA-007-EN)
Price: £44.95 (£52.82)
Antec SLK3000B Midi Tower Case - No PSU (CA-027-AN)
Price: £29.95 (£35.19 )
£675.27 for rather a lot of stuff, I'd say. Edit: I spent way too much time doing that and now it ends up just being a repeat.
