Time for a Server and Building a Server specs

Its time to upgrade and get a server.
Workgroup of 4 computers ( 3 running XP Pro and 1 running Media Center 2002 )

Here are my components of the network
-Dlink DIR-655 DHCP Router **This will be replaced WITH Netgear FVS336G Firewall/Router** (on the way from newegg)
-Netgear FVS516 Switch
-Netgear ReadyNAS NV+ includes 4 Seagate 500GB ES drives setup as xRaid (xRaid is netgears version of Raid)
-Dlink DNS-323 includes 1 Seagate 250gb drive this unit is used just for msicellaneous stuff
-APC SmartUps 1500 VA link: http://apc.com/resource/include/techspec_index.cfm?base_sku=SUA1500X413

What i would like to do with the server is,
1- Host my companies Email (not sure what i would need to do this, we send and receive about 300-500 emails daily)
2- Host a MS Access database which contains our products and our customer orders. In this case i need to have MS Office installed, which i have.
3- Backup of all computers in the network to the Netgear ReadyNAS NV+ (im planning to buy Symantec BackUp Exec System recovery to do this)
4- the server will be running some scripts to automate the processing of our order system in MS Access throughout the day. such as downloading order files, uploading files to various FTP locations.
5- Im planning on building a SQL Database.


Parts i currently have and will use in the new server build are
1- Adaptec 3405 Raid Card currently installed in a system running 2 of the 4 seagate drives bellow as a Raid 1
2- 4 Seagate Barracuda 160gb 7200rpm 8bm Cache Model number ST3160812AS (should i use these or upgrade to ES drives)
3- Intel Server Nic Model EXPI9402PTBLK ( on the way in from Newegg)

What i was thinking of doing was buying a Supermicro barbones system either one of the 4U systems, too many to choose from an list, or this one http://www.newegg.com/Product/Product.aspx?Item=N82E16816101073

With
1-  2 Intel Xeon E5410 ( did not buy yet)
2- 8gb of memory ( did not buy yet)


My dilema and questions
From what i understand Windows SBS premium will only function with a max of 4gb memory. so basically its pretty much useless that i buy any more than 4gb and install it on the server. In this case would it be better for me to buy Windows Server Premium?
With that in mind now that i have to buy Windows Server Premium, now i have to go out and buy SQL, MS Exchange, Outlook. and what else? would be a heck of allot easier if they would of just made a version of SBS that can handle more than 4gb of memory.

Another question is: regarding RAID, what would be the best raid configuration for the fact that i want speed of accessing the MS access database, reliability, and in case of failure i want do not want any downtime, system needs to keep running till i hot swap the failed drive(s).
Since i really want performance and reliability and in case a single drive fails, would it be smart to setup the OS on a differrent array and then the MS Access database on another? What are some suggesstions on how to correctly set up the server OS on a raid system? I hear so many people instaling the OS on its own Raid array or partition so im consfused about this. I just want to be protected in case a drive fails or something goes wrong with the system. I guess if something goes wrong wouldnt i be able to just restore it using the Symantec Backup Exec backup file stored on the Netgear ReadyNAS device?


I would like to set it up as a domain too. and leave the system i build some room for expansion, if i bought SBS then im stuck only buying 4gb and no room to upgrade the memory. But what happens when i decide to upgrade to Windows Server? do i still have to buy the inidividual MS exchange and all the other good stuff that comes with SBS?

last thing i wanted to buy is this APC enclosure to house it all in http://www.newegg.com/Product/Product.aspx?Item=N82E16816225011