I can't resist ... "it depends!"
If you're going to be serving up MP3s and not much else maxing out your DSL line is going to be easy - assuming anyone visits your site.
If you're going to be serving up plain, mostly-text, community-oriented OpenACS pages then you'll have a very hard time attracting enough visitors to max out your DSL line, at least if you buy one of the higher-speed options you've listed.
Perhaps a better way of thinking about the question is to set a budget
number, then to start thinking about trade-offs that get you a decent combination of performance and reliability within that budget. RAID 1
(mirroring) is nice. SCSI is nice. RAM > 1/2 GB is probably excessive, at least until proven otherwise. Dual CPU machines are nice but probably overkill for your situation. Hot swap is nice, fairly expensive, and not really necessary unless your under-the-kitchen-table DSL server is running a very mission-critical service, in which case it should be located in a datacenter with more disincentives to casual burglary anyway.
With prices the way they are today, a nice motherboard with integrated SCSI and NIC, 1/2 gig of DDR, a pair of fast SCSI platters mirrored by software RAID, and a couple of decent dual Athlon MPs are going to set you back a couple of thousand bucks.
And you'll probably be able to play quake on it simultaneously without your website visitors ever noticing ...
Working backwards, a single-processor machine with the cheaper non-MP Athlons and 256MB RAM will save you a few hundred bucks.
