Hey FILDOR,
I have a Dell with the 9700TX. There are only slight differences between the 9700TX, 9700, and 9700 Pro.
It is clocked at 263/263 as compared to a 9700 at 275/275 and a 9700Pro at 325/325. The other difference is that the memory on the 9700TX is a slower type of memory, so it will never be able to achieve the same clocks as a 9700 Pro. Other than that, all three cards are identical.
I have flashed the card with an unlocked BIOS (see the rage3d.com forums) and have been able to successfully overclock the card to be a bit better than the 9700, although I want to put some ram sinks on the card to ensure that it doesn't get too hot.
The performance of the card is awesome. There really aren't any games out there that can push the 9700TX. The most taxing game that I have played is NOLF2 at 12x10 with all of the settings cranked to the max and 4x AA on. The card still gets 70+ FPS. I get around 13500 in 3DMark 2001 to give you an idea.
I personally don't think that the small clock and memory difference is worth the extra money. I am very glad that I got the 9700TX. If I were you, I would save the $300 Can. and spend it on an R350 later this fall when Doom3 is out and you might want to upgrade anyways...