OEM is about the same price as the Upgrade windows 7. It is the best alternative to buying the full version if you cannot legally use the Windows 7 upgrade (you can still use the upgrade without issue but it is technically not legal so naturally I am not suggesting it). Just a note about going quad for extra performance with music recording - quad is important for more than that, nowadays it is ideal for gaming too, you don't want anything less (well, you could get away with X3 but definately not Dual Core)
The OEM license restricts you to your PC components I believe - so if you replace it you cannot reuse the OEM windows. Whereas the full license gives you more flexibility. It is not really a big deal as it will unlikely ever be an issue and even when it is one - given that thefull retail version is not far off the price of 2 OEM versions, then by the time you build a new comp or replace many parts in your old one you can just buy a new OEM version.
The 955 is practically identical to the 965, so most go with the 955 as I believe it requires nothing more than a single tweak in the bios to make it a 965.
Here is a build that can be done near budget
AMD Phenom II X4 955 Black Edition 3.0GHz - $159.99
http://www.newegg.com/Product/Prod [...] -_-Product
ASUS M4A89GTD PRO/USB3 - $149.99 + $2.99
http://www.newegg.com/Product/Prod [...] -_-Product
G Skill 4GB 2x2GB RAM - $109.99
http://www.newegg.com/Product/Prod [...] -_-Product
SAMSUNG Spinpoint F3 500GB 7200RPM 16MB Cache - $69.99
http://www.newegg.com/Product/Prod [...] -_-Product
Sony DVD drive - $24.99 +$1.99
http://www.newegg.com/Product/Prod [...] -_-Product
HAF 922 - $89.99 - $9.99
http://www.newegg.com/Product/Prod [...] -_-Product
Combo XFX 5870 and XFX 650w - $444.98
http://www.newegg.com/Product/Comb [...] mbo.421101
Windows 7 Home Premium OEM - $99.99
http://www.newegg.com/Product/Prod [...] -_-Product
Total: $1164.88
That leaves $135.12 left for monitor, so sadly we have overspent, but you said open to suggestions so I put an ideal build. You could save $15 reducing HDD to 500gb though that is not really worth it. I have given you one of the best gaming GPUs, you could reduce that to a 5850, though due to the combo the card was in you would save about $50 rather than $100, but still, an extra $50 would put you in range for a nice monitor if you were willing to sacrifice the GPU.
A cheap but pretty nice 24 inch monitor is Acer 2ms response time - $209.99 + $11.99 shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Or a cheaper monitor which may allow you to keep the decent GPU is LG 21.5 inch (LG monitors are damn good these days) - $169.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Given that you need a monitor in the price, it gets tricky to convert to Intel - you would want at least the i5 750 which is $199.99, and if you want USB3.0 and possible XFire you want the $189.99 mobo.
Talking of mobos, you may be able to save some money on the mobo I picked - I am not overly familiar with AMD mobos so can not really strongly recommend one