It depends. Do you need a SSD right now? Is it something thats going to make a real big difference in your use? 3 years from now are you going to be thinking "damn, why didn't I just spend that money on the 2.93, I can get a 4X bigger SSD now for the same price"? Or are you going to be ok about buying new processors and replacing them yourself (if that is indeed possible, I'm confused on that issue)?
The way I look at it, the money in the short term is best spent on the stuff you will be stuck with for the life of the computer. Thats why I went for the 2.93. I didn't touch the RAM, HDs or optical drive because I am planning to get 2 WDs in raid 0 and I am also planning to get either 6 or 12 GB (depends how quick prices drop) and when apple supports blue-ray I'll probably buy a blue-ray drive... but all those things are just getting cheaper. In a year 1TB drives will cost what 640GB drives do now. The RAM will obviously drop now that other systems will be using it. The processors might drop, but that gives you 2 options. Buy nothing now and wait 3 more years (or just wait forever), or buy new ones yourself but I really don't want to tinker with my MP until 1) someone has proven it works 2) a really good speed increase is available 3) there is a well documented step-by-step to make sure I don't break it 4) the machine is out of warranty. #4 is guarantied to happen some day, but 1,2,3 may or may not ever happen.
The only thing I did upgrade now that I could have/will later is the 4870, simply because it was $200 for 2X+ the performance.
Oh and I'm really not a fan of SSD. Couldn't care less about it personally. It's too new, over priced and unpredictable. If it were mainstream for the last 10 years you would know how it holds up over time. Right now I wouldn't trust it for long term use. And if I wanted to pay $6 per GB, I'd take a time machine back to 1999 (20GB HDs for $100).