I want to get some stuff for the 8 but i dont want to spend it all at once, so the plan buy the parts and pay the intrest for couple of months.

i need to keep a bank balance just incase i loose my job or something.

so my principal amount is 1105 or 2300 if i pay in 4 months how much would i spend on my CC with 7%

I think you need a amorization calculator and you can use or find those online.

does ne1 just know the interest formula??? i usta know it back in college.. but of course who retains anything from college

balance times interest rate divide by 365 times number of days in the month

$1105 for 4 months at 7%= $280 with $16 total interest.
$2300 for 4 months at 7%= $583 with $33

Use this website. It should have everything you need.

http://www.commerceonline.com/financial_calculators.cfm