i have Intel DG45ID Mobo and it has DVI port
http://www.trustedreviews.com/images/article/inline/8374-inteldg45idiopanel.jpg
But My Samsung 933sn 18.5tft has only VGA port
and I Using a VGA to DVI converter
is It Necessary To Replace this monitor to DVI ported LCD ????

no, adapters are fine
DVI Will give a slightly better picture but its not worth buying a new monitor over

I can confirm jbennet's comment. There is no need to change monitors

jbennet is right; adapters do not sacrifice enough quality to make you need a new monitor.