The HD4850 should be passable for 1680x1050 but if you want an upgrade in your situation I would sell the HD4850 and grab a GTX 460 myself. It should be great for 1680x1050 for a long time to come, especially if you overclock the card which it can handle easily and well(usually 30%+)
You could get a second HD4850 but then the question becomes your power supply. Even if the power supply can handle two of the cards or if you replace it then there is the large difference in power usage. The crossfired HD4850s use a full 60w more at idle and 80w more under load which is a very large difference. If you leave your computer on all the time like I do it will likely cost you $3 or more per month on your power bill which adds up significantly over a year or two.
If you prefer ATI or want to keep open the option of a multiple card setup in the future then another option is the HD5850, although it is a bit outside your budget. Perhaps selling the HD4850 can make up for that. There's one on sale for $270 right now;http://www.newegg.com/Product/Product.aspx?Item=N82E16814150477
But they have been as low as $250 on occasion. That card is nearly twice as fast as your HD4850 but actually uses slightly less power under load and much less at idle.