I always look at
http://www.xpbargains.com/dell_coupons/index.php/deals,
mainly because I am used to the way the deals are organized.
Others have recommended dealcatcher.com or slickdeals.net.
Example codes:
4J1M7748R2RRV6 is for $350 off Inspiron desktop $999+, limit 4000 uses, expires 11/21
6J5ZZ?ZX?RGJF1 is for $350 off Inspiron Desktop $1049+, limit 4000 uses, expires 11/29
Basically you start with a configuration that has slightly less than what you want,
then add desirable upgrades, like more memory, to push the price just above the
minimum required to apply the coupon. (It's the "starting price" that needs to be
pushed above the minimum. Then coupon will replace the tentative "instant savings.")
Some of the deals are click through deals, where you don't go directly
to the Dell page. Instead, you start at the bargains page and the link
takes you to a Dell page with some discount already built in.
If you don't find what you want at a good price, just wait until a future week.