You can't use both nVidia codecs at the same time.
If you already have nVidia PureVideo, you should be able to select them in DD.
When the nVidia PureVideo codec is in use with DD, just open Video Settings window and double-click on nVidia PureVideo in the list, a new window will be opened. In this window you can activate material acceleration (DXVA).
But as your video card is an old model (not optimized for PureVideo) I think, you'd better use the old 2.55 codecs.
_________________
Forum PCTVSat : http://pctvsat.free.fr
-----------------------------------------------------------------------
Pinnacle PCTV Sat + Terratec Cinergy 1200 DVB-S / Athlon 64 3500+ - 2048 Mb DDR - GeForce 7900 GS
Twinhan Vision Plus 1030A + Asus Europa II Hybrid / PIV D920 2x2.8 Ghz - 2048 Mb DDR2 - GeForce 7600 GS
PCTV Sat Pro PCI (PCTV 450i) + DVICO Fusion HDTV USB / Athlon XP 3200+ - 1024 Mb DDR - GeForce 6600GT
-----------------------------------------------------------------------
Collective installation with 2 LNB Quad and multiswitches DiseqC - LNB A = Astra 19.2°E - LNB B = Hotbird 13.0°E - Commutation = 13V/18V/22K + DiseqC 2.0