If you can afford to break 100 only to spend less once you get your rebate, the 8800GS is actually an upgrade in terms of performance. It is also cooler and quieter.
If you can't go over 100 at all (even though there is a rebate to bring the final price below 100, then I think the 8600GTS that Sylvanas listed is a good option. As an added bonus, it comes with a $30 mail in rebate Which makes the final price (after paying for shipping and getting rebate back) only $68.
That said, the gaming performance is lackluster. You will get a much quieter and cooler card, but you will also get a slower card.
If you are willing to take a performance hit, you may want to just consider using ATItool to underclock your x1800xt by 10-15%. It may make all the difference you want and it won't cost you anything.
I am an X1800XT owner and I've been wanting to replace mine for some time. The noise and heat don't bother me (it's in an antec p180 which helps to keep the noise down, and I've got decent airflow, also it's in my basement rec room so it's not like I have to hear it when I sleep. My issue is that the card is artifacting when it gets above 70C. I dropped the clocks by 5% and increased the fan speed and the artifacting is 90% reduced. I'm paying off some CC debt that I accumulated during my overlap from when I bought my new house and sold my old townhouse, so I still have a bit of waiting to do, but my balance is 5K less than it was over the winter, so I'm not doing too terrible.