Hi Mirage_Man,
You are in a difficult situation similar where you are limited to an exchange at the store you purchased your mattress which may not have any good quality and value mattresses. Post #66 here and some of the other threads it links to talk about this in more detail and have some suggestions about some of the options that may be available to you. Exchanging for a mattress that has the least possible amount of lower quality polyfoam possible in the comfort layers and then adding your own comfort layers in the form of a topper can be among the better solutions in this case.
Adding a topper to the mattress you now have may also be a reasonable option but it would depend on the details of the layers that were in it. If your mattress contains too much lower quality polyfoam in the upper layers then with your weight even adding a high quality topper may not help as much as you hope and may only be a temporary solution anyway once the lower density foams start to soften underneath the heavier parts of your body.
If you can let me know which store you purchased from and which mattress you purchased (a link would be great) I can take a look and may be able to give you some suggestions or thoughts that are a little more specific.
I should also mention that all new mattresses will go through an initial period of softening in the first few weeks and you may also go through some kind of adjustment period when you change from one type of mattress to another so even if adding a topper to what you have is the direction you choose to go it would be worth waiting until the initial break in period and your own adjustment period has stabilized before deciding on which topper may be best.
Phoenix