Actualy, I'm not too sure about overkill. I typically build database servers with quad Xeon processors and 4 GB RAM. Proxies are about the same. Exchange servers are usually duallies with a few gigs.. Of course, I cater to a FEW more customers than you probably do, Watts..
I'd go for the Athlon - The cost difference is fairly small, in the larger scheme of things and it will give you much better performance. If you've got the dough, consider a Tyan Tiger motherboard (About $200) with a single Athlon MP. The combo will be about $150 more than a typical Athlon/KT266A setup, but it'll allow you to add a second CPU later if you need it.
I've always been a fan of RAM - The more the merrier. A lot of apps will cache things in RAM and give you much better performance. If I had to pick between a system with a gig and a 10K drive and a system with 512 and a 15K drive, the 1Gig/10K would win every time, IMHO.
Then again.. If you've only got one or two users, you're probably not going to be doing big database queries as you're actively using Exchange as you're actively browsing the Internet. CPU is likely going to be the least of your worries. RAM and disk I/O will be more important.
How about this - Tell us what your database is like, how many users you expect and what speed your Internet connection is. Also, what's your budget. We'll then be able to make some more informed recommendations, rather than shooting from the hip.
- G