Windows Server 2003 and Exchange Server 2003 Licensing

I need some advice regarding MS licensing, hopefully someone can clear this for me.

I am going to upgrade 5 years old HP ML350 to a ML150 G6, 450 GB mirrored 15rpm, 10GB memory.
We currently have:
1 x Exchange server 2003 OPL
1 x Windows 2003 Server OPL
35 x Windows 2003 CAL OPL
35 x Exchange OPL
also had purchased 10 exchange CAL which is not in OPL (we did not know it was open license)
I am  going to upgrade the server to a newer one so I was wondering which path would you guys advices,
a - Upgrade everything to 2008 OPL, exchange 2010 OPL, costs more then £5000
b - buy OEM, we have 50 users, SBS 2008, and 9 cal packs, around £2300
c - Upgrade Windows 2003 std to enterprise (is this possible) so I can use 10GB memory
d - Use the existing software with the new server - Win2k3, Exchange 2003, server only with 4 GB.
Any advice be much appreciated!
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If you are not going to grow more than 25 users this year I would go with buy OEM, we have 50 users, SBS 2008, and 9 cal packs, around £2300.

Since SBS 2008 is 64Bit you will take advantage of all the RAM and the feature set you get with SBS you cannot beat for the price. But if you are use to enterprise product take an hour or two and get used to how SBS is installed and how you configure it. If you install this right it makes life so easy for you and your users. Remote access is a dream.  

Make sure you give you C partition a good 100GB or more.

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I should have been clearer. SBS can take up to 75 Users. If you out grow that you can move right into Windows Essential Server.
I should have said Migrate not Upgrade.... Sorry
_etoptasAuthor Commented:
Thanks RickEpnet, we have done number of SBS 2008 installation - thats way i was considering to choose OEM path, most effective and cheapest solution.
Will just wait and see if anyone has different poing of view on this.
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