There are some advantages to using an Extreme:
- there's only one ethernet port on the Express, which will be used up connecting to your cable modem, so your network will always have to be wireless. If in the future you want to add network storage, it would be better to be wired.
- You can't connect a hard drive (at least officially supported - not sure if people have workarounds for that) to the USB port on the Express.
- The port on the Express is 100Mb/s while the Extreme is gigabit, which could slow throughput (although that wouldn't be a problem in a wireless only network.)
On the other hand, the Express is half the price and provides the same wireless performance as the Extreme. I would look on it as a good short term solution, with the view of getting an Extreme in the future. You would always find a use for the Express if you upgraded to an Extreme.
One more thing you should think about - since your Dell supports only wireless G, you'll need to set up a mixed G/N network, which will slow things down, although the N connection will still be much faster than G. I would consider keeping the Belkin router and using the Extreme as N-only bridge to it.