Depends on incoming links, if urls look nice, lots of unknown factors etc. I consider Google XML sitemaps a help to get listed (faster) but not as a guarantee.
Sitemap just help your site get indexed easily and properly...I dont think it helps in bringing up your page in the search results...For that you need to increase the PR and have quality and Keyword reach content at the same time...
Lats time I did that the site slowed so much (die to continuous spidering) that in the end I deleted the sitemaps from google and resubmitted smaller sitemaps.
You can control / throttle most website crawler / sitemap generator to some degree to avoid overloading the webserver.
We can use the sitemap in such way, it will provide us to connect it with in short period of time, but it would not depend on the url links.
sitemap only helps u tell the spiders about your links they won't help u in getting rankings in serps.. ranking depends on other factors.