O2 + ODAC.
A pretty transparent combination, just under your budget if you can DIY.
I have the O2 and it powers the HD 650's very nicely. I have my gain set on low and I'm able to hear good sound. I can't recommend the ODAC yet as it's still on its way in the mail. Don't really know much about the Fiio's all that well but a lot of people here seem to go with it. Probably can't go wrong with either one.
As far as portability, if you decide to take the O2 on travel you'd probably need a backpack or bag of some sort because it will be awkward carrying it around. Its size is roughly 3.5 x 4.5 x 1.5 inches.
Hope that helps!
I would suggest the E17 route for now (unless you have an android phone that supports USB Audio [Cyanogen Rom, Samsung S3, ect] where the E7 may be a better option) which will allow you a more diverse upgrade path. You can add the E09k for a more powerful SS amp (E7+E9 was the rig i used for a long time for the HD650s) or utilize the L7 for a line out to another amp of your choice.
As for me, I eventually used the E9's line out to a Shiit Valhalla (which was very good for the price) then graduated to Violectric V200 (which I currently use w/ Kingrex's DCA and LOVE).
I would like to reiterate that the price range you are at isn't optimal for the HD650s. If you are really put off by the prospects of spending 2-3x on amps/DACs for the 650s, may i suggest the HD5x9 line. It's much easier on gear requirements while still sounding fun.
What kinda' music are you listing too?
What USB would you recommend buying? I have an ODAC on the way from JDLabs and they are including a USB with it. Should I be getting a better one just in case? Please post some links if possible.
I think he's referring to the USB Port, not the cable. The Monoprice cable included works just fine. The build quality seems much better than most cables you'll find bundled with devices.
Edit: Yay! 1000 posts!