Building the computer yourself would be the best option, but since you really seam to want to buy a Dell anyway.....
The dual 4850 512MB setup is generally a better option but as games consume more and more vRam soon 512MB will hinder such a setup so going with a 4870 1GB now would be a good option. Also you can just add another 4870 1GB down the road for even more performance. As it is another 4870 1GB would only cost you $145 so it makes sense to add the second card yourself and not have to pay that insane $300 "upgrade" price.
Personally, It hurts to hear the prices of the "add". I will be answering your question,
Instead of the usual " HEY NOOB BUILD IT ITS CHEAPER"
It's a rude remark as it doens't answer your question, but true that it does benefit building.
Anyways, Gts 240 2048mb you say? What the hell is a gts 240? Well googling it real fast i think it was discontinued, i know of the gts 250 tho.
You also say tight budget, but willing to pay 100$ MORE is not a tight budget anymore by the way, tight is $20-30.
Depending on the games you play, is what you'd want. personally i game, but i don't see the reason to dual.
Well if you wan't go for the 4870 then, it's awesome performance at a great value. cheaper then the gtx 260