I'm deciding between the Casio PX-110, PX-120 or the Williams Encore digital piano. I tried a few of the keyboards at Guitar center and narrowed it down to these. I wouldn't mind paying a little more and get the PX-120 if the 128 polyphony makes a big difference. My current keyboard is a basic 61 key Yamaha which is my first and only keyboard so I would like to upgrade to 88 weighted keys.

I don't need any of the fancy stuff like the usb connection or flash drive on there. Just something basic and sounds great. Any suggestions appreciated. Thanks.

PX110 will do fine. If you don't use layered sounds, and are not very advanced, you won't notice low polyphony.

Spend the savings on a bench - you'll need that much more than polyphony.

The action of the PX-120 is, in my opinion, the reason to get it over the other models you are considering. The sound is also better (imo). The high polyphony # on the PX-120 is also a guaranty you will never get notes dropped (and you will feel less the need to upgrade any time soon).
In addition to the bench, consider a good (stable) stand that will put the keys at the right height, and a pedal as well because the cheap plastic ones are not satisfying at all. They can't stay in place and are nowhere close in feeling like a regular piano pedal.
To me, if it fits your budget, the PX-120 is a no brainer.

Thanks for the replies. Actually I have a piano bench and a table already. I will get the sustain pedal that looks like the piano kind.

Make sure it is compatible with the DP you will get. There are no standards.

I'm sure the table will fit just about any keyboard, made it myself, its a table rather than a stand. The bench, I measured a real piano bench and made one, even has padding.

The PX-110 has the ZPI while the PX-120 has the AIF. Does anyone know the difference between the ZPI and AIF sound chip?

Here is what I get from the Casio web site.
AIF stands for Advanced Intelligent Filter technology, is the Casio's latest sound processing system. It is supposed to sound "more realistic".
I don't know what ZPI stands for. They describe it as 3 elements sound ship.
Casio Digital Pianos (DPs) with AIF offer the 3 layers piano sounds as the ZPI equipped ones (PX-110, 310 ...).
The Casio web site says about the PX-110 "Three different dynamic levels (samples) per key"
The AIF DPs have 180 piano samples 11 MB stored in ROM (180 might be 60 x 3 levels).
In comparison, my Yamaha P-120 had 22 MB of Piano samples. If I compare the Piano#1 of each DP (PX-320 that I now own vs. P-120), I would say that they are both very good, with the P-120 doing a better job on sustained notes (you can hear the loops more on the PX-320). I would bet the PX-120 is the same.
AIF seems to apply to the voices/timbres and not just piano.
Casio refers to sympathetic resonance capabilities only for AIF equipped DPs.

One important more important thing between the PX-110 and PX-120 is the action. And you need to try - there is no substitute - to know which one you prefer.
I tried the PX-110 and I did not care for it.
Then I tried the PX-120, and I considered replacing my 4 years old P-120 with this lighter Casio. The keyboard on the PX-120 is silent compared to the PX-110. And it feels better as well to me.
I ended up with the PX-320 because of a less than $550 delivered deal (gone) on the Internet. I would have been just fine with the PX-120.
The USB, Registration slots, SD Card reader, more advanced recorder and additional voices on the PX-320 are bonuses that I and my little one will probably use in the future.

The ZPI chip is the sound source of the older Casio Privia DP. It is also used in the WK3300, WK3800, and WK8000. I am impressed with the sound quality of the ZPI vs the AIF. I had been comparing a PX-575 which has the ZPI with the PX-120/320 which have the AIF. They are in close proximity in the Guitar Center I sometimes go to. I like the piano sounds better on the PX-575. I think they sound richer and fuller. Unfortunately, this piano has only 32 units of polophony and with 620 tones, layering just a few tones, notes drop out quickly due to the limited poly. If you want to layer sounds, the 320 is the best bet, unless you look at Roland.