Atom = Not worth it IMHO. You'll be noticing slowdowns all over the place, and probably in certain cases an Athlon XP will be faster.... so Atom wouldn't be much of an upgrade. My suggestion:
AMD 7750 Black Edition - $79.99
NCIX.com - Buy AMD Athlon X2 7750 Black Edition Dual Core Processor Socket AM2+ 2.7GHZ 3MB 65NM 95W Retail Box - AD775ZWCGHBOX In Canada.
Biostar 780G Motherboard - $79.44
NCIX.com - Buy Biostar A780G M2+ SE AMD780G mATX AM2+ PCI-E 2PCI SATA2 RAID Video Sound GBLAN Motherboard - A780G M2+ SE In Canada.
Kingston DDR2-800 1x2Gb - $29.99
NCIX.com - Buy Kingston ValueRAM KVR800D2N5/2G PC2-6400 2GB 1X2GB DDR2-800 240PIN DIMM Memory - KVR800D2N5/2G In Canada.
Total = Approx. $190
Reasons: Its not worth getting the 5200+ when the newer 7750BE is not even 10 more expensive and can be OC'd a bit with the stock HSF. The onboard 780G graphics on the Biostar board will be capable of some very light gaming, as well as watching HD videos without issue. Kingston ram = lifetime warranty and inexpensive.
OR
Get this:
NCIX.com - Buy Shuttle XPC K45 Barebones Mini PC LGA775 GMA950 DDR2 1PCI SATA VGA Sound LAN -FACTORY REFURB- - K45-K1-BK-V1-REFURB In Canada.
With any mid-range 775 CPU (e2xxx series) and some ram (the above mentioned Kingston stick would work nicely)