Adding a page before NewForm and linking modified NewForm to a list

ok ladies & gents...
i'm by no means a genius with sharepoint designer.  i've been playing around with it to become more familiar, and now i have a question!

so... here's what i'm trying to achieve:
i've created a modified NewForm called NewFormMod.aspx for a list.  i've added the capability whereby a text field will auto-populate with a value that's passed in the url.  (see image #1 below).  this is working fine.

i've also created a blank web part page.  i'm going to add a DVWP that displays customer data from SQL.

what i need help with:
1.  how to i add the new web part page so that when someone selects "New" from the list menu, it takes them to this web part page that has my DVWP on it?
2.  the user will interact with the DVWP and locate the customer they want.  they will then click on the customer number.  when they click, i want it to take them to NewFormMod.aspx i've created, and pass the customer number in the url so the text field pre-populates.
3. i haven't yet figured out how to get NewFormMod.aspx to be associated with the list, so that when "ok" is selected, it adds the record to the list.

when all is said & done, the process should work like:
user selects New from list to add an item.  the web part page with the DVWP should be displayed.  the user interacts with the DVWP to find the correct customer, and they select the customer by clicking on the customer number.  this brings up NewFormMod.aspx with the customer number field pre-populated.  the user completes the rest of the form, and when they click OK, the data is submitted to the list.

i don't think it matters, but just in case...  this is WSS 3.0
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