Your system is actually pretty good. I have a 4870Toxic and i can still play most games on max settings (1280x1024).
Well for that budget, and since you already have the amp, i think that you can get a great headphone and a great soundcard/dac.
Spending around 220€ -sorry about the euros- on the headphones (For instance the beyerdynamic DT880) and 180€ on the soundcard (for instance, the asus essence st/stx) you can get a killer sound system for gaming and music (in fact, those two togeter are refered as the "holy grail of gaming sound" in some thread around here, wich btw, is quite informative if you game a lot, and still, the Essence soundcard is considered one of the better quality soundcards for music and overall best sound quality over here).
There are loads of other options, but you probably are going to get those recommended a lot in Head-fi (btw, note that i only write based on others opinion, i do not own that hardware, but i can tell you that i have spent like a year reading these forums).
Depending on how much you game or listen to music you can get more music-centered or game-centered hardware (music centered often giving more sound quality at a better price (but with less gaming features) and game centered bringing game features like 3d-to-stereo virtualization and other stuff). The Essence for instance is designed mainly for music quality (in wich it is told to excel) while remaining great for gaming. You can get other cards (as the Forte) wich will give you more features for gaming but not such an high-end audio quality. Most of the dacs dont bring gaming features (with some exceptions) so they focus on sound quality and you may find great sounding ones cheaper than the soundcards.
For what you say about your father, maybe you are more interested in speakers than in headphones (i mean, if you both are going to be listening at the same time). In that field i cant tell you much, just to ensure that you get surround compatible hardware if you are going to be using 5.1 or 7.1 (etc) setups. (For instance, the Essence is mainly designed for headphones, although you can still plug speakers on it).
If you also want to switch all that harware from your pc to his laptop, then you will have to get only external hardware. External soundcard and/or dac(probably usb) to your amp to the headphones is the only solution that comes to my mind (well, you can add a lot of other stuff to that chain, but its not necessary). You wont be able to install an internal soundcard (as the essence is) in your laptop) and since the laptop probably does not have a quality built-in audio output, getting usb connected hardware lets you bypass all the internals of the laptop to get a clean audio signal (and less interference from the pc since the external units probably will be self energy supplied). The only drawback of the usb is that you cannot connect it to other internal soundcards or portable audio devices (like an ipod) wich dont bring usb plugs (wich i believe its all of them).
If you are not goint to use the laptop as much as the pc, you can always get a good soundcard for the pc, and then a dac wich you could plug both to your soundcard (so, no usb) and your laptop, and only switch the dac and your amp to your father's laptop. You will still get great audio quality, and since i presume that you wont be gaming seriously on the laptop, not having gaming routines its not a problem (they arent much of a deal anyways)
So, sumarizing.
- Dacs and soundcards do pretty much the same thing (if they are quality soundcards), wich is giving a cleaner sound signal and better sound quality. From there you just have to ensure that they have the plugs that you need (headphones, 7.1 or whatever you want to use) and maybe if you want any special gaming feature (cmss3d, dolby headphone, etc. not really needed unless you are a hardcore gamer, and still, not really needed) (as a personal tip, the dolby headphone effect is also used in music and movies, and while it is a matter of preferences, you may like it. I like what it does to music, sounds more natural and real, but it makes the instrumentals a bit louder than the voices).
- Amplifiers only provide the power needed by your headphones to reach good volume levels (wich in some cases means audio quality too, since you cant appreciate the details of the audio if you cant provide enough volume). This is not a real concern for lower impedance headphones (around 32-80ohms) but it is for cans over 150ohms (well, that scale varies depending on the quality of the amplifier). So a 32 ohm headphone will sound better out of onboard or a low-power amplifier than a 600ohm verison of the same headphone. And since i dont think that the difference between impedance in headphones of the same model is really that big, you should not bother to get into the high impedance world of wasting money (on expensive dacs and headphones) just to get tiny differences. Again, this is a matter of taste and budget (with maybe a little snob factor on it).
Sorry for my late-at-night english...
