RAM is only part of a video card. Just like a PC with DDR3 RAM does not make it faster then one with DDR2. You simply cant compare video card specs from different series of cards.
The 9800GTX+ has a much better core clock then the GT240 and is a solid step up from it in terms of performance.
Your 300w PSU will not be able to power either card. You would need to upgrade it or look at the ATI 5670 which can run on the 300w PSU and offer GT240 performance.