Well, just for the saxe of clarity, I should stress that a DAC - a digital-to-analog convertor - is necessary whichever approach you use. The differences lie in the implementation of the DAC. So, broadly speaking, if the sound sound source is a computer, you have 3 choices:
1) Internal soundcard, with DAC on the card, outputting audio via line-out
2) external soundcard, with DAC outside the PC, connecting via USB (in pro-audio circles, sometimes via Firewire)
3) connecting a computer via its digital output to an external hifi DAC, rather than USB. Hifi DACs don't tend to have a USB option, though some are now cropping up, an example being the Russ Andrews
If you use the internal soundcard's DAC, then you have the interference problem... it needs to be very well-screened, and as you note, in many cases only a proportion of the investment goes into the DAC. An external card shouldn't really have the screening problem, but it may also be packed with gimmicks. A hifi DAC is more likely to concentrate on pure sound quality, and you are likely to be able to plug all your digital hifi sources into it so they all benefit.
However, many hifi DACs do not offer a USB input, and some contend that connecting a computer via say the optical interface isn't quite as good quality. The other problem is that, while in America say, there are a number of cheap-but-good hifi DACs to choose from, in the UK I only know of one: the Beresford.
An alternative route is to use the DAC of an existing high-quality component, IF it allows input from other sources... some use the 384kHz upsampling DAC on the Cambridge CD players for this purpose but, again, not cheap.
If you want to stay around a hundred quid, you therefore have 4 options, not 3, lol
1) Use an internal soundcard with good screening
2) use an external soundcard, which may be a bit pricier features-wise, but which has better screening
3) Plug the optical output (or coax, if appropriate) of the computer into a component with a decent DAC
4) Buy the Beresford, which will also potentially upgrade other hifi components, especially the more budget components.
The computer soundcard scene is constantly in flux, and to be honest I'm not up-to-date with the latest-and-greatest. Because I have an interest in audio production, I tend to use pro-audio external soundcards, which tend to be more focused on audio quality than 3D sound effects for gaming, etc., and these can be had quite cheap, but aren't necessarily that "hifi" in terms of a "musical" sound: they tend to be designed for accuracy for monitoring purposes, rather than to flatter the sound, and a proportion of the expense on THOSE tends to go into getting good sound INPUT, for recording, which may be a waste for you.
You see, it's an awkward question. That's why you haven't had many replies, lol...