Your 9800GTX+ provides more or less the same performance of the GTS450. I had one, and it was an excellent performer up to 1280x1024. For 1680x1050 it simply didn't have the muscle. Your card have to provide pretty decent performance with lowered details at that resolution, or lowering RES to 720p. My Samsung 2032W which max RES is 1680x1050, accept 1280x768 quite well.
If you definitely want to change your GPU, you don't have too may choices. Go for a HD 6850 or 6870 if you find a cheap one, or wait until Nvidia release its GTX 600 mainstream cards.
The price range you are suggesting are not going to get you the type of card you need to full fill your rig. That is a nice rig and you should have a nice card to go with it. I will show you some cards and some benches.
The cards you should get are the 560 ti or the 6870 or even the 6950 if any are left? The bench marks for all the cards posted above and what I suggested. I hope this helps you and I wish you the best on your choice. Good luck
Thanks to everyone for your help on this topic. Video cards are definitely not my forte, so kudos to all who know their stuff. I've decided to wait on one of the nvidia 600 series to see what is offered for the price. This should allow me to budget more on the card, maybe between $250 - $300. If none of the 600 series cards sound suitable to my needs for the $$$, I'll most likely go with the 560 Ti or the HD 6950 and wait a few years for what's to come. Since gaming isn't my highest priority, I think either way I'll benefit. Thanks again for everyone's help!