My current computer is much like yours. Just didn't build it myself. Got one of the first Dell XPS's (which they renamed slightly after to the XPS 435mt).
Intel Core i7-920
600 GB HD
ATI Radeon 4850 HD
After a few years it ended up having
24 GB RAM (when I got it, it was so cheap to buy went ahead and maxed it out)
120 GB SSD (SATA 3 but of course it doesn't support sata 3 so I have to stick with sata 2 speed)
and a ATI Radeon 6850 HD video card (which is the highest model this computer it can take)
Still runs great, but those little things bug me, and just like you stated, doesn't have USB 3, memory speed and has the old PCIx stuff. Added to that the lack an easily replaceable power supply, inability to take a faster video card, and in general no expandability options at all, I'm currently building my next computer as I write this
New computer will have
Core i7 4790k
250 GB SSD (and all the old drives)
NVidia GeForce GTX 960
And a case that's easy to add and upgrade items as I please
I'm also building this thing for absolute silence most of the time. All the parts have fans that won't spin up until a temperature threshold is reached, after that they spin up but stay quiet and really I won't care since I'll probably be gaming, transcoding, or doing a software build.
I can't wait til I'm done!