Applogies, but this is going to be a heck of a long first post

We've got a small/medium sized client that has (at present) a single X-Serve running snow leopard, which provides the entire range of internal network services for the office--who are exclusively on Mac. We've got everyone on Open Directory, and running the followig services:
1) File Shares (via AFP)
2) Home Folder sync
3) Daylite Database services
4) DNS
5) DHCP
6) VPN
Due to the nature of the work (media, mostly) the poor thing is being pounded flat by sheer volume of data being moved around via AFP. We've optimized as best we can, stripped down home folder sync inclusions etc, but the reality is that the hardware just isn't strong enough to move the data.
Now, being a NFP organization, budget is a serious concern. Because we are also a Dell authorized dealer, the absolute best bang for buck was a Dell PowerEdge with a big stack of 146 and 300gb 15k SAS drives for about 1/2 the price of a not-even-close dual-drive x-serve--even including Windows server 2008. Granted, the integration won't be quite as simple as with bunging in a single X-serve, but even counting the additional labor time, we come in significantly below the cost of going with another X-serve.
Now, I can get this thing up and running--more-or-less. We've got a simple mock setup in the lab running off a Super-Duper'd snapshot of the current config, so I can sandbox with that nicely.
MY big question is the most efficient and stable way to set this up, so I was looking for some input from people with more cross-platform experience than me

The way I see it, I have few options:
1) Dirty and ugly, but functional: Two separate user authentication systems; OD for initial sign on, leave the home-folder sync and other mac-specific services on the X-Serv, move all other filesharing activity onto the W2K server, have separate (but parallel) user list to provide sharing permissions via SMB.
2) Find some way to have the X-serve provide all authentication services, have the 2k8 server provide the majority of the storage services, leave everything else on the X-serve.
3) Create a new AD on the 2k8 server, have all the mac clients log into the Domain, leave mac-specific services on the X-serve, but have the 2K8 control all the security aspects of the network.
Of all of the options, I really like #2 best; which leaves each server performing the duties best suited to it; There really isn't anything wrong with the X-serve; it's just not set up or built with the right hardware to be a hardcore file server. Trouble is, while there are plenty of resources for migrating Windows Server TO mac, there is very little in the way of information on how to make them work properly together... Of course, it may just be that I am not asking the right questions...

Any input would be greatly appreciated!