I had been running vector games with the default line thickness, brightness, etc. at 640x480 for around 6 months and I wasn't happy with the results. I recently changed ti 800x600 and honestly, it made a big difference. It still isn't as good as a real vector monitor (of course), but it is definitely a noticable difference. Don't know if it'd be worth a large expense or not (I don't play many vector games).
The pictures posted in this thread really understate the difference in appearance between 640x480 and 800x600.
Wade