General requirements - Go big!

Seriously it's hard to guess on hardware until you know what the load is. Is this a brand new system? Any prototyping to get an idea of reads/writes/transactions/sizes of queries?

Raid - I've used 5 for years and been successful, but there are good reasons to not use anything other than 1 or 10. http://www.baarf.com

NICs - Can't imagine needing more than 1 these days. They're so fast. I guess a spare would be handy, but I've never really worried about it.

CPU - I'd go quad core these days, doesn't make sense to go less, but are you 32 or 64 bit? Do you have SSIS packages, linked servers, etc? 64-bit can be flaky in some places. However 64-bit usually wants more on chip cache over speed. Hard to tell on this load if 2 or 4 would work. Or if you'd need 8. Depending on budget, I might go a 4way with 2 CPUs for now. Or an 8way with 4 CPus, giving me room to expand.

RAM - more is better. If you can get 8 or 16GB, get it. Even with 32bit.

Clustering - this is about downtime. Do you need it? It's expensive?

Make sure your version of SQL supports this. Most higher end systems require Enterprise, so be sure that your company is buying that.