SLI? tbh I'd say no. Depends on how long you intend to drag your parts out for. The theory is when your PC gets a bit showing it's age you buy another card slip it in, they're much cheaper then too, and poof speed boost. The reality is you tend to just buy a brand new current top line card. Also SLI doesn't work in every game, and can have it's own issues. It's a great option to have further down the line, but you have to decided if you'd use it or pick up a GTX 960 instead.
As Malicious said it's not exactly a coke vs pepsi thing. There's wriggle room on price n performance in some areas. Worth checking Staysicks Tomshardware links up top on what's good for budget. I'm unsure if you'd need 6-cores though. Only now are things taking advantage of 4 cores. Source Engine supports 6 though as of a few months back That's maybe something to google. I think the 6-core stuff is more for video editors and such than gaming.
For RAM, since I've now seen your current specs, you'll want more than 4GB. As for capacity vs speed, it's another balancing act but I'd say there comes a point where RAM could be too fast for your needs, but you can never have too much room. However don't go too slow. I have 6GB, but it's slow. The capacity is nice, but sometimes things lag a little bit. (It's also DDR2 though, so much slower than the slowest DDR3). For next PC I'm looking at 8GB Though I do video work and 3D modelling. I also have a ton of background programs open. Which really seems to offend the folks yet to adjust to the RAM explosion a few years back with Vista. Also as malicious highlighted, it's the timings that can help with speed, not so much just the mhz.
Well going back on the SLI stuff up top, buy the single card now. If you buy two cards right away you're fucked for wriggle room later down the line. Also I'm pretty sure the 450 were mid-range sop despite it adding up to 900 and having 2GB of RAM I have a feeling they'll be slower. They definitely will be in games that don't support SLI.
I've never bothered with SSD's. Still unsure if I will. I'm hard wired for capacity. So 60GB vs 2TB for the same price, I'm buying the 2TB. Maybe I'll look to see what the budget can burden. It's pretty much only going to affect boot up times though. My files will still be stored primarily on the HDD's and my Steam games total about 250GBs, so they're definitely not going to fit. SSD is up to you. I personally think they're still too expensive for what they offer. When it's about 1/4 the capacity of HDD for the price (so that's a 500GB SSD for £100) then I'll look into them. But Win 7 boots up in like 30 seconds or so anyway. I tend to go get a drink or whatever while it's booting so it's not time lost.
Also you're only giving it 10 days? I'd get it up before 1st November, RMA could take 2 weeks then you'll miss the launch. But yeah if it's going to be another 6 months almost then I'd spend that time just brushing up on what's important n what not cos all of those are going to become obsolete by then. Or at least much cheaper I guess. I'd strongly recommend more than 4GB RAM though. That's my only main concern.