Trying to get plex working on my home system.  It just about works for SD content, but really really struggles with HD.

I have my desktop running the plex server, connected via ethernet to my modem (TG589vn).  My media is stored on a seagate goflex home 2TB which is also connected via ethernet to my modem.

my TV and PS3 are in a different room, connected via Netcomm Powerline (ethernet over power) 200Mbps.
Playing HD content to my PS3 direct from my seagate works fine, but the PS3 won't play all types of content, hence the use of plex.

Because it is PS3 it needs to use DLNA with my computer transcoding rather than a proper Plex application (like on a Roku)

my computer specs are:

AMD Phenom 9500 Quad-Core processor 2.2Ghz
4GB RAM
NVidia Gforce 8800GT
Windows Vista home Premium

I believe it's my processor that is struggling (as when I try and run the media, and bring up task manager it shows processor at 100%), and whilst I know it isn't the best, even so I would have thought quad core AMD 2.2Ghz would have no problem transcoding HD content

Is it the processor, or am I missing something else obvious?

Would the best solution be to upgrade my processor, or would I solve it by getting a roku or low-end smasung blu-ray with plex app or something to remove the need for my PC doing heaps of transcoding


ETA:  what is wierd, is that when I use Airplayit,  play something on myipad and airplay to my apple TV, it plays HD absolutely fine, even though I am fairly sure that is still the pc doing the transcoding (I could be wrong about that though)


ETA2: playing on my ipad via plex app, then airplaying to my apple TV seems to work fine too, but it doesn't support 5.1 which is annoying. also I like to be able to fiddle on my ipad whilst watching TV, not have it sitting there draining battery