For web design, i have used an old AMD x64 (4200+ with slight overclock) for years and that's fine with:
- Many ineternetExplorer, firefox, chrome windows open (+ debugging windows),
- Several text editor windows open (or dreamweaver if you choose),
- Several largish photoshop windows open,
- Youtube playing me tunes,
- Outlook,
- Torrents seeding
- FTP connections,
- Remote DB Connections,
- + general PC use,
- And all while powering 3 monitors via an old 4850 and onboard graphics.
Basically any half OK dual core will do!
(in fact if you dont plan on multiple bowsers and windows a P4 would probably be fine

)
I find a good amount of ram helps with multiple browser windows open and photoshop (or GIMP) (especially if you are heavily using jQuery or similar). I use 6GB of DDR2 @ 800mhz....
If you plan on running virtual servers, maybe an intel i5 or AMD FX
(my x64 can run virtual servers, but they can be a little sluggish at times)
At times my x64 can struggle, but that's only when i have s**tloads of windows and programmes open, any i3/i5 or FX should hammer all you can throw at it for webdesign!