I hate 640x480. I think it is a waste of a perfectly good arcade monitor. The whole point of having a multisync monitor is to be able to handle multiple resolutions. If you are going to just leave it at 640x480 you should buy a 31khz, or VGA, monitor and be done with it. They are usually cheaper than a full multi-sync.
Additionally you don't need soft 15khz or an arcadeVGA if you just want to run in 640x480. Just about any video card made in the last 20 years will support 640x480 and you can just select it from your video modes under your display adapter in Windows XP.
I set MAME to make my graphics card switch resolutions to whatever game I happen to be playing at the time. This way I always get native resolutions, which look better than anything else.
With an analog monitor this means that the image will get shifted around a bit between resolutions. That does not mean the game will be unplayable. In fact, you may not even have to adjust it at all.
There will be an optimal setting that will work best across multiple games. Some game may not fill the whole screen, and others make have some of the screen cut off, but they will mostly all be playable.
Additionally, you can use software to adjust the placement of your screen. MAME has slider controls that allow you adjust the placement of the screen somewhat, and Advancemame has even more options.
I have a multisync monitor. It remembers 3 resolutions. Every time I switch to a third resolution I have to resize the screen. The controls are right on the front of the monitor, and it takes me maybe 1 minute to get it right. If I did a quick job I could do it in 15 seconds. I don't mind this at all and gladly take the inconvenience to get the image quality of native resolutions.
Both of those monitors come with a little control board you can put somewhere on your cab. Most people put them right underneath the monitor behind the control panel.