I'll go the opposite of Doris... especially in regards to the ISO.. The D40 sensor is just SO much better than the D40x that the ISO 200 is actually much better than ISO 100 on the X. With a higher ISO, you will also take crisper shots because of a higher shutter speed and less camera shake.. with better results than ISO 100 on the X.

I'd buy the D40 personally and save the money on a better lens. In fact, I'd buy the D40 used and save even more! The D40x improved a couple things and took a step back in a few others...
The D40x is 95% the same as the excellent D40, with the following changes:

1.) ~$200 more expensive than the almost identical D40

Put that ~$200 towards the new Nikon 55-200mm VR or the SB-400. Bouncing the $129 (list) SB-400 flash makes a huge difference in photo quality indoors, and megapixels don't.

2.) Slower default ISO: only ISO 100 compared to D40's ISO 200. This can lead to images blurred from subject or camera motion or reduced depth of field as mentioned above. The higher-speed sensor of the D40 allows excellent results at ISO 200 while the less sensitive sensor of the D40x requires ISO100 for the same thing.

3.) Slower 1/200 flash sync, down from the excellent 1/500 of the D40. This reduces flash range outdoors, makes it difficult to get sharp flash shots of sports action, and reduces battery life with flash.

4.) Larger files sizes clog hard drives, memory cards, waste backup CDs, and take more time to transfer, copy, backup and archive. You'll get fewer photos per memory card, which isn't much of an issue today, but waiting for all the other things is. The MP difference is only really important with cropping and 6MP is plenty for that.

Just my 2c's... If you can find a used D40x (rare) for the same price, I'd go with that.. but all-in-all, I'd just buy the D40. Keep in mind that with either the D40 or D40x you are sacrificing a ton of used lens options out there since they won't work on this drive (no drive screw)... may not be an issue for you but just thought I"d mention it.

BTW, I own a D50 (backup) and a D80 (main) and love them both..