Although the Nikon D3000 and Canon XSi are both nice cameras, in my opinion you shouldn't overlook the Sony A330 and Pentax K-x.
First, lets look at prices for each of the 4 cameras at B&H ( http://www.bhphotovideo.com ).
D3000 w/18-55mm lens, $549.95 with a $50.00 "Instant Savings".
XSi w/18-55mm lens. No price at B&H. Their listing says its been discontinued by Canon. A forum on Amazon says the same. Apparently the XS and XSi are being replaced by the T1i and T2i.
The T1i lists for $799.95 with a $100.00 "Instant Savings".
Sony A330 w/18-55mm lens, $549.99 with a $50.00 "Instant Savings".
Pentax K-x w/18-55mm lens, $513.99.
The A330 has several features not found in the D3000. First, the A330 has arguably the best Live View of any current DSLR. Second, the A330 has an in camera focusing motor. Third, Sony has their version of Image Stabilization (IS) in the camera body. The D3000 doesn't have Live View nor does it have an in camera focusing motor. If you want auto focus you have to buy Nikon AF-I or
AF-S lenses. Nikon has their version of IS (called VR) in some of their lenses but not all so you pay for it over and over. With the A330 any lens designed for it and meant to auto focus will. Plus, every lens used becomes, in effect, an IS lens. The A330 uses the venerable Minolta Maxxum "A" lens mount introduced in 1985. A Maxxum AF 35-70mm f4 zoom or 70-210mm f4 zoom will auto focus on the A330 and be an IS lens. The Sony web site lists over 100 compatible lenses.
Advantage A330
The T1i has Live View and video capability. Canon, like Nikon, has their version of IS in some, but not all, of their lenses. Again, you pay for it over and over. In my opinion, Canon has created some confusion with the EF-S lenses since they are NOT compatible with their full-frame (5D, 1D) DSLR cameras. So a person who begins with a T1i or T2i and adds 2 or 3 EF-S lenses has to buy all new lenses if they decide to upgrade to a 5D or 1D.
The Pentax K-x offers the following for a mere $14.00: Live View, video capability, double/multiple exposure capability (a feature seldom found on cameras costing 2 or 3 times more), an in camera focusing motor and Pentax's version of IS in the camera body. Again, all of those features are missing from the D3000. The K-x uses the "K" lens mount first introduced in 1975 so any lens designed for that mount is compatible. Although there are limitations when using a 35 year old manual focus lens it can be used and becomes an IS lens. Pentax lenses designed for their long discontinued AF 35mm film cameras will auto focus on the K-x as they should.
Advantage K-x over all 3 competitors and especially over the T1i since the K-x is $185.96 less.
So you decide for yourself which camera gives you the most for your money. In my opinion its like deciding whether to buy a car with a manual transmission, no air conditioning, an AM-FM radio with no CD player (D3000) or paying the same for a car with an automatic transmission, air conditioning and an AM-FM radio with CD player (A330) or paying $14.00 (K-x) more and adding a sun roof to the car with the automatic transmission, air conditioning and AM-FM radio with CD player.
If it were my money it would go to the Pentax K-x.