Shard servers -- go big or small?

Hello everyone,

I'm designing a website/widget that my business partner and I expect to serve millions of hits daily. As such we must shard our database (and we're designing with shards in mind right from the beginning). However, the one thing I haven't been able to figure out from Googling is the best hardware to go with for shards. I'm using exclusively InnoDB tables.

We'll (eventually) be running 3 groups of database servers:
a) Session servers for php sessions. These will have a very high write volume.
b) ID servers. These will match a couple primary indices (such as user ID) to a given shard. These will have an intense read load, plus a moderate amount of writes.
c) Shard servers. These will hold the bulk of the data. These will have a high read load and a lowish write load.

Group A is done as a database instead of using memcached so users aren't logged out if a memcached server goes down. As the write load is high, a pair of high performance master-master servers seems obvious. What's the ideal hardware setup for machines with this role? Maxed RAM and fast disks seem reasonable. Should I bother with RAID > 0 if I have a live backup on the other master? I hear 4 cores is optimal for InnoDB -- recommendations?

Group B. Again, it looks like maxed RAM is recommended here. What about disks? Should I go for 10K or will regular SATA2 drives be okay? RAID 0, 5, 10? Cores? Should I think about slaves to a master-master setup?

Group C. It seems to me these machines can be of any capacity because the data they hold is easily spread between shards. What is the query-per-second per dollar sweet spot when it comes to cores and number of disks? Should I beef these machines up, or stick with low end hardware? Should I still max the RAM?

I have some other thoughts on system setup, too. As the data stored in the PHP sessions won't change frequently (it'll likely remain static for a user's entire visit -- all variable data can be stored in Group C shard servers), I'm thinking of using a memcached setup in front of the database and only pushing writes through to the database when necessary. Your thoughts?

We're also starting this on a minimal budget (of course), so where in the above is it best spent? Keep in mind that I can recycle machines used in Group A & B in Group C as times goes on.

Anyway, I'd love to hear from the expertise of the forum. I've been reading for a long time, and I'll be writing as our project evolves :)