I've also read on some audiophile forum that piping the music through any PC introduces noise and loss of quality. Not sure what the options are in that case other than a dedicated media player of some sort and then you lose the MC library interface and all the flexibility thereof.
With a few exceptions (see below) a digital signal is a digital signal. Well designed software (which JRiver is) does not introduce any noise or signal degradation into a digital signal. Think of what JRiver does with an external DAC as being analogous to moving a file from a harddrive to flash drive: if the process is working correctly the transfer is bit-perfect. PCs are hostile EMI environments, but that is only really meaningful in an analog context (i.e. if you have a DAC inside of your computer), and even then, many in-PC DACs have outstanding performance.
All that said, there are some possible signal issues with digital transfers (such as buffer underruns or clock jitter):
Buffer underruns (drop outs or pops) occur when the DAC is not being supplied with data as quickly as it is playing it. Depending on your system speed and the DAC you choose, this might be a problem, but JRiver has adjustable buffer settings to address this kind of issue (including loading the entire file to memory if necessary). This is not a subtle problem (you'll notice it right away if you have it), but should be easily fixable with the tools JRiver provides. I very occasionally get these with a four year old laptop when the processor is under heavy load, and I only mention it because you're using an older laptop. If you don't do extended file analysis while you're listening to music, this shouldn't be much of an issue (unless your hardware components don't get along with each other in a very serious way).
Jitter is more complicated, and subject to a lot of debate, but see this article for some useful info on jitter and some measurements of jitter in a few common DACs (including the Fiio E7 mentioned above) http://nwavguy.blogspot.com/2011/02/jitter-does-it-matter.html. The punch line is that jitter *is* potentially audible (especially at high levels), and that there are several methods that DAC designers use to address the issue. Many modern DACs (especially once you get into the "over a hundred dollar" price range) have jitter so low that it should be inaudible (although claims about audibility or inaudibility are almost always hotly contested).
There are differences of opinion about some of these issues, but I wouldn't worry about interference or signal degradation as they're commonly meant in the external DAC context.