If you do not live in a high density area, 900Mhz will suffice. 2.4Mhz is supposedly better because it is "less crowded". If you live in the city in a crowded condo, the 2.4Mhz is a safer bet.
I live in an apartment building in the city. I just bought a Panasonic 900 mhz, very basic, but seems to work fine. I have a thirty day return or exchange policy and see a different Panasonic on sale for less, and it has a digital answering machine and speaker phone. It is also 24ghz.
Both are selling at BestBuy for around $30.
On the web I see the 24 listed for $70 to $100 at other stores. Maybe the 24 would be the way to go?
Also, 2.4 may not be a good idea if you're running - or planning on getting - a wireless router in your house...the signals interfere making your connection disconnect each time you use the phone.