I have usually owned Dell computers and have mostly found them to be reasonably quiet in the past (especially if I put in a fanless video card) but I recently bought an xps 410 and had to return it because I thought it was just too noisy in comparison to the old Dell I have now.
I'm rather surprised at that - a relative of mine recently bought a Dimension E521 (AMD X2 4200+), and it's very quiet indeed. Maybe not SPCR-fanatic quiet, but certainly very acceptable for an off-the-shelf system.
I've no experience with the XPS range, but you can spec the Dimension E520/521 up to an E6600 or X2 5000+ which should be enough for most people's needs. I'm not personally a great fan of Dells, but if they've served you well in the past you could probably do a lot worse.