If this is 64 bit windows then the amount of Ram on the GPU is not an issue.
Did you install your motherboard's drivers? Chipset first, and I do GPU drivers 2nd.
[edit] I think i see what you mean, this card is not a powerful GPU for Adobe's GPU accelerated functions. You also want to make sure GPU acceleration is turned on in photoshop.
Lastly, of the 4gb 240's I found, most used DDR3 ram instead GDDR5.
What was the old card that felt faster?