A few months ago, I got a dual-slot 9600 GT to comfortably fit and efficiently work in my XPS 400.
I will say this though...I also now have a new computer, as the CPU bottleneck (I had a Pentium D 930), was more than annoying. Specifically, while some games that were more gpu bound and/or well optimized for two cores, Bioshock for example, ran pretty good. Others howerver, ones that were more computer bound and/or primarily utilized one core, like Oblivion, were played with annoyance - the bottleneck was very obvious.