HSU is the other company I wa talking about for an entry level hometheater sub.
I looked long and hard at the STF-2 and VTF-2, but decided the SVS PB-10 was what I wanted.
I agree with Pixle's SLI opinion.  I'll try to elaborate a little.
Ok, lets say you want to get the most bang for your buck and you're not just doing SLI because it's the newest thing in town and it's a cool concept.  I'm also just going to look at nvidia for now in the interests of showing just the difference between SLI and non SLI.  ATI has the X800XL in this range too that is a pretty nice option.
If your budget is $500 to $600 for video and motherboard, your choices are
1. SLI motherboard and two 6600gt cards
2. non-SLI board with a 6800gt 
3. SLI board with 1 6800gt with the intent to add a second one at a later date to increase performance
Out of these, option #2 is the best performer most of the time, and cost is less than the SLI option with two 6600gt cards.  Two 6600gts are going to run you $400 while a single 6800gt is $380-ish.  An SLI motherboard is also going to cost you more ($50 or so).  
So, if you go with 6600gt SLI, you're going to pay more and get less out of it.  You're also going to need a more expensive powersupply to power 2 videocards.
I put option 3 in there because if you really want SLI, I think that would be the thing to do.  6600gts in SLI is just stupid in my opinion, while 6800gt cards in SLI will be faster than any single card.  Presumably you'd add another 6800gt later on when the price goes down for them.  I don't think this is such a hot option either, as ideally you want the exact same type of cards in SLI mode.  I think you even want the same revision number to have an exact match.  If you wait a year or something to get another one, you might not be able to find a match.  Plus at that point, there might be a single card you could run that would outperform 6800gts in SLI.  There are 512mb cards coming out soon etc.
If your budget for video and motherboard is like $1000, then I'd say go to town on SLI.  Get a couple 6800gt cards right now and get amazing performance that no single card can match.
It seems like the $1000 to $1500 budget you mentioned though would prevent you from going that route considering just about everything seems like it's going to get replaced.
What kind of ram to you have actually?  If it's some pc3200 you might be fine just keeping that.
If you keep the case you have now, don't need additional storage or optical drives, and can keep your ram... then dual 6800gt might be an option for you.  If you take $1000 or so for the video / motherboard, you still have enough to get a nice A64 and powersupply.
EDIT: I think I'll add this...
I looked over the SLI article again writing this but didn't look at the "conclusion" they reached.  I can't believe they said that two 6600gt cards is a better option than a single 6800gt.
In just about every single case, a 6800gt outperformed the 6600gt SLI cards when the settings were turned up (which is what you're going to be doing if you spend $500 or $600 on your video/motherboard solution.
At the worst, I think you could say 6600gt SLI is about the same as a 6800gt.  If you then look at it from cost perspective, the 6600gt SLI still doesn't make sense.
Ok... I'm done now unless you have any questions about what I said or think something isn't correct.