Best Headphones For Use With A PC (Music Listening)...

Hope you guys can help me with my conundrum.

I recently built myself a high end PC and am using the onboard sound outputs of the motherboard (a Core i7 Asus P6T Deluxe) which I am told is supposed to be pretty decent, but having been on Mac's for years I have no idea whether I should in fact be looking for a separate internal sound card instead or is the inbuilt motherboard stuff actually decent these days?

Anyway, I have no headphones at the minute but my neighbours have just had a child (with another on the way) so my speakers are having to go lower and lower every evening to the point where by I just wanna splash out and start listening to some tunes again and so now want some decent headphones.

My first question obviously is whether or not you think I should invest in a separate sound card or not. Second question is what headphones are currently the best in your views for the £200/£300 mark and thirdly, given my musical taste which is largely R&B, Hip-Hop, Pop Music i.e like Jacko stuff (mainly lots of big beat, bright sounding music) would your recomendation change for something different?

A couple of years back I had a pair of sennheiser hd600's and really liked them but had no idea whether I was getting the maximum out of them given what I have been reading here on the forums about some headphones needing additional amplification.

I would appreciate any help you can offer though as basically I have a blank canvass, don't mind throwing a sensible amount of money at it but want my music on my PC to really blow my socks off. If I need a sound card, again, any recomendations would be great and I thank you all in advance for the advice that you are likely to give...

Thats some great advice, thank you drubbing...

I didn't think I was getting the maximum possible from my old HD600's and so that in many ways is why I thought I would ask the experts here.

How much in which case would a decent amp to drive these things be? Also, turning the coin on the other side slightly, what is currently the sweet spot set of headphones to use in this sort of setup "without amplification". I.e which ones will hit their peak performance best without amplification.

I think perhaps it might be a good idea to maybe listen to the best headphones (with amplification) and then perhaps the best without, just to see is if I need to spend the extra £££ and indeed to see if I can actually hear the difference...

Although still only 28, my ears arn't quite what they used to be in my latter teen's / early twenties  lol