At the end of the day, it depends what you need.
I know composers who work in purpose-built studios with Mac Pros running full Pro Tools HD systems including their favourite digital mixer/controller and booths for recording vocals and instruments.
I also know composers (and even 'up-and-coming' 'sound designers') who work from home on an iMac with a couple of cheaper studio monitors and an academic version of Pro Tools or Logic they bought when they were still in school (and have been riding on the cheap/free upgrades).
Both may be composers, and both will be paid for what they do, but both have incredibly different needs.
Similarly, I know editors who work on the latest Avid Media Composer in purpose built studios, with Blackmagic, Avid or similar video cards installed into their Mac Pros, driving three screens and a 55" client monitor. And I know editors who work on 2009 Macbook Pros on an old version of FCP7.
I know colourists who work off networked Linux systems with Resolve, Scratch, Baselight, Lustre or similar software in a theatre with a 2k cinema projector installed and calibrated, with $20,000 control surfaces.
I also know colourists who work from home on Resolve with a couple of monitors and an Avid $1,000 control surface.
All of these people get paid work, and some of the work is indistuingishable from one another (i.e. the great artists working from their laptops or home studios are not necessarily producing any worse quality images than those working in million-dollar studios), but all of their needs are completely different, and the computers and equipment they buy will reflect that.
What do you need?