Frankly, any PC manufactured 3 years ago could easily support a home-based Web server on a DSL line (i.e., 400Mhz Pentium II w/128MB RAM).
For a short while, I ran a couple of static page Web servers on my SDSL (symmetric DSL: 784Kb downstream, 784Kb upstream) that were averaging a couple hundred discrete visitors, tens of thousands of page views, and approximately half a gigabyte of throughput. There were days when it maxed out at double those figures.
Albeit I was running sites with static pages, but average page sizes were generally larger than any ACS served page. CPU activity was insignificant (so much so I ran a SETI@Home instance with the leftover CPU cycles). I'm personally a big SCSI fan, but any IDE drive could have served up the pages.
My system: 733Mhz PIII on an Asus CUSL2 motherboard (133MHz front-side bus), 384MB RAM, an Ultra160 root drive and some Ultra2 LVD drives. I have no doubt that I could have served up the site with my previous computer, a 400MHz PII on an Asus P2B-S (100MHz front-side bus), 256MB RAM, and some Ultra2 SE drives (the PC I built three years ago).
While I admit I haven't run Classic ACS nor OpenACS recently, I ran it on my old 400MHz PII system (and other similarly spec'ed PCs), and I believe your planned system is total overkill. Buy all the memory you want; it's so incredibly cheap nowadays.
