The Astroman

Hey guys!
I just bought a netbook, and I'm trying to view high-def videos on it (it has a 1366*768 screen), however, they sometimes lag.
Unfortunaly, I have a shared Intel GMA 950 chip, which isn't too powerful. I have 2 gigs of RAM and an Intel Atom N280 (1.66GHz).

1) What makes one HD video "harder" to run then another?
I can read some fine, but others lag. Is it the bitrate, resolution, etc.?
2) What is the maximum of the answer to 1) I can play on my netbook? (e.g. maximum bitrate 2000kb/s)
3) How can I optimize my netbook to play videos more smoothly (special drivers, codecs, settings, ...)

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Intel P4 HT 2.4 GHz // Asus P4P800
Radeon 9600// 2x256MB PC3200 Infineon + 2x512 MB TwinX 3200C2 Corsair XMS
80GB IDE//140GB SATA // CM Stacker
DVD Player // LiteON SHW 16H5S Lightscribe
Logitech Z5300// SB Audigy 2 ZS
Akasa Rheobus // 19’ LCD
Keyboard // Logitech MX518
FAN of: Lost,The OC, Alias, DH, Prison Break, 24, Nip/Tuck, CSI, NCIS
If it's not broke, don't fix it!