My current linux server is a Athlon X2 5000+ paired with 4GB of DDR2 800 RAM. This server have 3 main purpose : Mail server, File Server and DNLA server (Universal Media server, a PS3MediaServer fork).
The two first application are fine with this CPU, but CPU seems to bottleneck transcoding fr the DNLA part. It works when transcoding one feed, but if more than one feed need transcoding the video start to be choppy (buffering). I'm starting to think about CPU update might solve this. Motherboard is an ASUS M3A76-CM (supported CPUs can be found at http://www.asus.com/Motherboard/M3A76CM/#support_CPU).
This mobo support virtually every Athlon/Sempron/Phenom AM2+ compatible CPUs. The Athlon II 640 seems to be a good deal (65$ CAD).
I don't think RAM will bottleneck transcoding but I want your opinion on this.
HDDs for media library are two WD 640GB Black RAID 0.
DLNA Clients are a PS3 and a Raspberry pie with XBMC.
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