A very slow g card can slow preview but we are talking ancient here. My 'slow' monitors are powered by a quad head pci card that is only 2d and 8mb per monitor. Great for 2d but chugs on anything with video.
Any modern card will be fine, just about any 3d agp card ahould be fast enough. To speed up the preview you need a faster cpu.
Some editors can make use of faster g cars and some can link with very pricey cards deisgned to work in tandem with a particular editor (matrox rtx 100 i think, may work with premier), but these cards are well expensive and you really will only get the full benefot if you have a speedy pc.
A 2ghz p4 is about the minimum you can edit with, I started with a similar speed cpu a few years ago and could only preview in small windows at low quality.
I would suggest a pc upgrade.