With a laptop you need an amp. The D2000 is pretty efficient and low impedance, but generally needs a bit more current than most cheaper OPAMPs will provide. To me at least it was never satisfactory being run directly from any device that wasn't actually capable of more than the usual. (Just for instance, it sounds great on my Cowon D2 DAP and my Archos 43 "Internet Tablet", but sounded absolutely horrible from onboard devices, my PSP, DS, and etc.)
Note that the E7/E9 aren't just amplifiers, but actual USB DACs with amplification as a nice bonus. If you wanted amplification alone, the far cheaper E5 will work in instead of those two (though I'd imagine it's probably not the best matched for the D2000 for cheaper prices? I can tell you I wasn't happy with the CMOY using an OPA2132 however... I wasn't happy with the OPA2227 with any headphones either, but especially not with the D2000s. I can't say on any others available to the CMOY though.) Of course, an E7 or E9 should sound a lot better than the crappy builtin SPUs on most if not all laptops and probably most if not all onboard SPUs in any desktops (as well as a number of soundcards.) If nothing else, they tend to be less inclined to pick up noise from nearby components. Anyway, in other words, if you're actually wanting a USB DAC, you should probably go ahead with one that can handle headphones properly (since a line output isn't going to cut it anyway.) If you just want an amp, get just an amp.