Well you have to be specific because there are several versions of the GTX 260, including the 65nm with 192 shaders, the 65nm with 216 shaders, and the newer 55nm with 216 shaders. If you are referring to the XFX GTX260 Black edition that runs at 666 mhz core and 2300 mhz memory, then $230 (199 after rebate) shipped (via newegg) is not a bad price to pay...but considering you can get an MSI GTX 260 clocked at 655/2100 for $199 (170 after rebate), the price premium is not going to justify the small gain in clock speed. So most of us would rather just buy the cheaper version and overclock it ourselves, and spend that difference on a better cooling system for the card, which would increase overclockability as well as reduce temperatures and noise levels.
But the end result is what ever works out for you, if you prefer XFX as a brand (as many do), and you dont really care for doing any tweaking or overclocking yourself, then yea the extra few bucks isn't too much to ask.
As for the GTX 260 length, the card itself should be about 10.5" long, making it about as long as the 9800 GTX, if thats what you have now. The power connection for the GTX 260 is actually along the side edge, not the rear of the card, so as long as you can measure 10.5 inchs from the expansion slot bracket on the case, you should be just fine. But note that the GTX260 does run a bit hot, so you will need to have proper ventilation in your case, which would ideally be at least 1x 120mm intake fan and 1x 120mm exhaust fan, if this is just your typical HP or Dell system you are dropping this card into, cooling may be a concern you want to address.
Also note that the card is a dual slot card, so you will not be able to have any other expansion card sitting under your PCI Express graphics slot. If you currently have a sound card or anything else in the slow below your pci express x16 slot, you will have to relocate it. As for how this card will stack up next to a 9800 GTX, you can refer
here for a simple side by side comparison, or refer to reviews such as
this to see how it stacks up against other cards in this type of performance range (9800 GTX, GTS 250, HD 4850, etc).
But to sum it all up...its worth the extra money if you prefer the brand, like the included extras, and would prefer to have a factory overclocked graphics cards instead of doing it yourself. You wont get $50 more performance compared to a regular card, but that price difference will reflect in other areas such as the XFX double lifetime warranty and the included extras (games, flash drives, what ever they give these days).
hope that helps.