I usually when it comes to OEM towers like dell and upgrading CPUs I look at ebay at that model number and see what the fastest that was used.
I do that since Dell doesnt supply a compatibility list.
That fastest I saw was a Pentium D 3.4.
If you can find that cpu cheap (about 20 USD) cheap (try used on Ebay) then go for it but otherwise enjoy your new card.
maybe bump up your ram if your under 2gb.
Increasing memory can be the best way to improve performance.
If your running a 32bit OS then dont go over 3gb since it wont really use more than that.