Hi,
Do you have a merchant account already?
If you dont and you can process the transactions manually an easy solution is
http://www.mobileeft.com.au
You can look at paypal as well though I dont think you can connect it up for real time processing with 1 shopping cart, but you can send invoices, links etc and they can pay through paypal or accept visa and mastercard using paypal, depends oh how you wantto do it.
If you are selling physical product and you dont need real time processing you can use 1shoppingcart to collect the credit card details and process the credit cards manually.
If you have lots of transactions it could be better to have it done automatically.
A few choices and it depends on your circumstances.
1shoppingcart is good for linking everything together.
Some links
Good info link
http://www.ecommercewebsites.com.au/article-payment-processors.php
Practical example
http://www.bellaforma.com.au/shop/Payment-Options-p-11.html
Glenn