boogityman, edh and Steve have lots of good, relevant points.
Regarding noise, generally, PCs from big companies are much quieter than they were say 10 years ago, but not anywhere near "SPCR quiet". However, you don't have to guess, at least not with Dell systems. They have a pretty extensive anechoic chamber and acoustic analysis tools. I was in contact for a while with the guy who ran their main sound lab for many years. They were providing acoustic info mostly for server and office products, for the engineering types who'd specify their needs in great detail, but they have long since expanded the acoustic testing to include all their consumer products as well.
The acoustic data is buried not in the usual spec sheet, but the environmental data sheets. They keep moving the location of the main page for these, and currently can be found in the Environmental Data Sheets link on this page: http://content.dell.com/us/en/corp/dell ... ducts.aspx
The data sheets page, then, is here:http://www.dell.com/content/topics/glob ... ?c=us&l=en
Since there are dozens of products, and they vary in acoustic, let's look at what Dell says are best selling SOHO desktop PCs:http://www.dell.com/us/soho/p/desktops- ... redir=true
Among these, the $550 models look similar to something a DIYer might want to build. The Inspiron 660 Desktop -- i5 processor, 8gb ram, 1tb HDD, Win 7 in a case that looks like a small mATX. Look up the datasheet -- http://www.dell.com/downloads/global/co ... 11m002.pdf
At the top of page 4 is the acoustic data, based on ISO 7779 testing. The bystander position SPL is the closest to SPCR's measurement standard -- 1m distance from the object, with A-weighting -- which is 26.4 dBA in idle. The operator position is a closer distance (just over 0.5m), so it is curious that it is actually lower here: 23.7. Maybe the mic is facing a fan in the bystander position. SPL for various other states are given, and the highest practical number is for HDD accessing, which is 25.9 and 25.1
These SPL numbers are modest, but they are nowhere near what lots of SPCR DIYers achieve routinely, at least judging from what they post in the forums. Our lab systems are typically <15 dBA@1m. Ditto my personal PCs. Usually placed on the floor beside or under the desk, they are essentially inaudible in normal use, including high load or with HDD accessing. None of the Dell datasheets I've looked at over the years indicate this kind of quiet, not even close in SPL, nothing under 20 dBA, and there's no way they have the same smoothness we achieve -- by tuning out tonality and choosing the smoothest sounding fans -- & HDDs -- and leveraging the cooling headroom provided by superior heatsinks by running fans as slowly as possible.
So... don't fool yourself. Yes, today's commercial desktop PCs are pretty quiet, but except in a noisy office, they would always be audible to the user. Whether you find them annoying or not depends on you, of course. They are a bit cheaper for the same basic specs, but with careful shopping, A DIYer would end up with better quality parts, and far lower noise.