a Window box? ouch, this should be slightly difficult.
well, i have read your specs about your SGI O2 comp <A HREF="http://www.sgi.com/o2/index.html" target="_new">here</A>.
i think an athlon xp 2000+ or 2100+ cpu plus 1GB of DDR2700 memory on a eg A7V333 or a ABIT KX7-333 motherboard plus a 120GB 7200RPM ATA100 harddisk plus an ATI All in Wonder Radeon 8500DV should be better in terms of 3D power, image quality & capturing. obviously, all these hardware supported by the Windows XP Professional OS.
besides, if you are hard disk high power consumer (because of capturing & video editing), you can used a raid0 configuration with 2 120GB hard disks.
moreover, if you want a professional capturing graphics card you can have a look on the Matrox RT2500, the Pinnacle DV 500 Plus or the Pinnacle Pro-One.
& finally you can choice also a geforce TI4600 which is a little better for 3D than the ATI AIW R8500DV but without a most part of video features. in this case you can reenforce the A/V capturing/editing with one of the above professional capturing cards.
addendum
the TI4600 & the R8500DV are described on the THG <A HREF="http://www.tomshardware.com/graphic/02q1/020304/index.html" target="_new">here</A> and <A HREF="http://www.tomshardware.com/graphic/01q4/011211/index.html" target="_new">here</A>
<i>if <b>you know</b> <font color=white>you don't know<font color=black>, the way could be more easy ...