while i know about them enough to suggest them, i'm not an expert on the he-400i
you may want to read user reviews
http://www.head-fi.org/products/hifiman-he-400i
they are good cans, but soundstaging is not as wide as some other cans.
depending on your needs or wants there are a few ways you could power them.
want virtual surround?
-if your onboard audio is good quality and has virtual built in.. just use an amp
-use a decent soundcard if its powerful enough to handle the cans
-use a soundcard and aux out to an amp
dont care about virtual surround?
-use onboard + amp
-use external dac+amp
want an all in one solution?
-use a dac+amp combo unit
-use a soundcard if its powerful enough to support the headphone you like
some good products:
fiio e12 amp (pretty decent power output)
fiio e10k amp+dac combo (middling power output but good value)
fiio a3 amp (middling power output)
schiit magni amp + modi dac stack (can be used separately or together. amp has high power output)
jds o2 amp + odac dac combo (o2 is available without odac, or for extra cost with. decent power output but less than magni)
audioengine d1 amp + dac combo (middling power output)
creative z soundcard (virtual, decent amp strength)
xonar dx soundcard (virtual, weak amp strength makes pairing with external amps good, better DAC than z)
...there are more products but these are all fairly budget level and affordable.
honestly which fits you depends on your needs/wants and what product you have. now, i've heard the he-400's are not hard to drive but based on what i'm reading some people seem to be having issues with weaker amplifiers. not that they arent working but that the sound seems better on decent amps like the magni, o2 and similar.
--
want another option?
the dt990 is open, has a greater soundstage than the he's, similar bass quantity and is great sound for the budget as well as being very comfortable. the negatives are that they are bright headphones on treble and some people find they sound a bit cold (a warm amplifier helps). mids on the dt990 are recessed due to a v-shape.
both are good cans. if you're on a budget the 990's are certainlly stellar and have better soundstaging (you would want a good amp to power them like the e12 or magni or similar power output or greater) but if treble bothers you or you want something a bit more easy listening the dt990 has it.
depends on your preferences really.
there are of course other cans out there. i'm only listing ones i know enough on to list.