You may want to consider our checkout mod which puts the login and register options on the same page (among other advancements):
http://www.websitecm.com/x-cart-mods/xcart-easy-checkout.html
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To just edit that text and create a button, the file you want to edit is:
skin1/customer/main/anonymous_checkout.tpl. The language variable is: txt_order_loginpage