ok what i imagine is happening with your svideo is, it is only allowing you to output the video from the pc into your tv at 640x480 interlaced, at 60hz only. this is the problem with 'svideo out' on a pc gfx card, and why most people avoid it.
the video converter card you ordered from ebay will just do the same thing (i believe), it will just let you send 640x480 interlaced at 60hz to your tv (and possibly add some lag between your pc and tv, as well as some occasional stutter - depending on if it is a high quality device, or some unbranded cheap item).
anyway, if you want to stick with svideo (not recommended by most), then i would advise for mame that you run either mameuifx, or groovymame, instead of the regular mame release. these versions of mame when set up correctly will allow you to force every game you play in mame to be played at 60hz in 640x480 interlace, while remaining smooth and without sound hiccups. they do this by allowing any game you load in mame which does not run at 60hz by default, to be either sped up or slowed down so that it becomes 60hz.
sorry if this isnt the advice you were hoping for.