ATI HD 4870 or if you can price one the 5870 line.
Very cheap (relatively), superb gamer cards.
As for the GT250 being the same as your card but with more RAM: Don't do it. You've already got that card, and you're looking for an upgrade. You'd be wasting money. The amount of RAM on a video card is not what makes it fast. It's the entire cohesive product. They often tack on cheap surplus (i.e. slower) ram on older cards to market them as 1GB (or in some cases 1.5 GB+!) but this really does not help the card in any way. Sometimes it can slow the card down because the ram is slower.
I would look veeeeeery carefully at the specs of the GT250 if you really want to go that way. Scrutinize the RAM, the frequencies, the bit quality, what level DDR (2/3/4/5?) it uses, etc.
Much as I am a bit nvidia fan, these new ATI cards are really top dog right now. Cheaper, better, cooler. If I had the funds that is the way I would go right now.
EDIT: Checking here:
http://www.gpureview.com/show_cards.php?card1=571&card2=606They're almost 100% identical. It's just a product re-branding. Don't get it. you already have this card and it's not doing what you want. The same card with a little more RAM won't give you much of a boost. They even apparently have a 2GB version of the card, but again just tacking RAM on an older card to boost sales and make it look better.