I upgraded a family member's box running R5B7 on a Pentium III to R5E50 with the following:
# Video type: DVB-T SDTV and Foxtel Digital SDTV
# Type of system: combined front- and back-end
# Noise level: I can tell it's on until I play some media, without it being annoying. I'm not sure how good my hearing is
# CASE: 7 Year old beige ATX case
# PSU: came with the case above
# MOTHERBOARD: ASUS K8V-X
# CPU: AMD Sempron 3000 with stock heatsink
# RAM: 512MB
# CPU HEATSINK:
# NORTHBRIDGE HEATSINK:
# CD/DVD-RW: Liteon Dual Layer DVD burner
# HDD: 200GB Seagate IDE
# GRAPHICS CARD: Nvidia MX4000 PCI (I was going to upgrade to an AGP card if it didn't work well but it seemed good enough)
# SOUNDCARD: Onboard
# TUNER CARD: Hauppauge PVR-150, TwinHan DVB-T MiniTer
# REMOTE CONTROL: Dvico remote (came with a second card from my system)
# NETWORK CARD: 3COM 3C905-TXM
This was about the cheapest motherboard around at $45 Australian. It uses a VIA K8T800 chipset so I was a bit nervous about what has happened with VIA chipsets in the past but I already had a K8V-X SE so I tested the upgrade on that first to make sure it would work ok. The only trouble was it had a different network adapter. I couldn't get the onboard Marvell adapter working even after a kernel recompile so I just threw in the 3COM adapter from the old box, which worked straight away.
I had to apply the patch at
http://svn.mythtv.org/trac/ticket/3022 and recompile because the backend was crashing. This may not have been necessary if I had defined all the input connections in one go. I had both cards installed through mythtv-setup but only had the input connection for the twinhan defined at first. I also had to patch the /etc/init.d/mythtv-backend script to get rid of the pid file if it exists on startup, as has been well documented in the R5E50 installation hints thread.
This is using ACPI wakeup.