Building a balanced rig -
http://www.tomshardware.com/reviews/balanced-gaming-pc,2477.html
This will show you the comparative cpu/gpu performance. What I took from this was that AMDs current crop of processors are faster than the current crop of GPUs need them to be to avoid a bottleneck.
Games (requires 100% resource utilization of a modern pc) and music (needs the guts of an ipod shuffle), are polar opposites as far as requirements go.
Since games and playing music don't require or really benefit from more than 2 cores at this point, a quad will give you little over a dual core. Since playing games and music at the same time is hardly intensive multitasking, a quad will give you nothing there either. Since a L3 cache is worth 200mhz of performance, generally, and mhz for gaming is king, then you should go with the AMD 550, imo. Plus it has a ~75% chance to unlock and OC to a 3.4 quad core... an option you might want later.
If you buy a ddr2 board that allows crossfire as an option you plan to use later., then down the road realize you need to but lack the ability to upgrade your ram because ddr2 is gone, or nvidia comes out with a face pwning 99ghz 4d gpu fridge magnet for $99 or something..., you will not be happy. When you realize the crossfire on that board you suggested is limited to dual 8x, and that anything beyond two 5770s will saturate the bandwidth of the bus and place a hard cap your performance, you will not be happy. You'll dump your ram, and your mb, and be out $220 worth of stuff.
If you buy a $70 non crossfire board you plan to upgrade later if you need to, then you will need to dump your MB, and be out $70 worth of stuff... while getting all the relevant port, socket, and whatever else upgrades on a board that probably allows for crossfire and SLI, with a pci bus wide enough for your future GPU upgrade.
You said you didn't want to touch the config for three years, and won't ever overclock, but just having the options of an easy 1 minute core unlock to a 3.1ghz quad core, or having an easy upgrade path are things you will probably want in the future.
The L3 is dependent on many factors and may or may not be of benefit. As to the 200 MHz, take a look at this and you will see how dependent some things are on L3 and others are not affected at all.
