Yes. You can connect any number of sensors in addition to a WiFi shield, subject to the obvious limitations of
Available physical ports,
power supply availability
processing power
Non interference of Wifi (usually 2.4 GHz) and 432 MHz equipment.
None of these are 'show stoppers". It's mainly a case of applying applied common sense.
Physical ports should be a non issue. The 432 MHz unit should require as little as one pin to communicate (eg half duplax async) and at worst should need no worse than something like TXD, RXD, enable, power/enable.
Power supply availability from solar power is just a matter of design. If this transmits autonomously and/or "gets in touch" on a timed basis and does not have to receive on demand then standby power can be low. If it has to monitor a receiver then you will have to tradeoff receiver always on simplicity against receiver occasional wakeup and listen with lower power but more complex scheduling. This is standard fare for this sort of application.
As a very very very rough indication
- PV panel Wattage ~~= 12 x Average Wattage x days standby ability.
better
Sunshine hours can be established via the wonderful Gaisma website - this link is to the page for my home city Auckland.
The 4th chart / table down shows sunshine hours by month. Fpr auckland
Insolation, kWh/m²/day January - December
6.55 5.88 4.85 3.56 2.62 2.06 2.24 3.06 4.17 5.10 5.97 6.51
Best month is January with 6.55 hours and worst is June with 2.06 sunshine hours. Most sites world wide give about 2 hours worst case - which is why the first formula above use 24/sunshine_hours = 12. If you are in NYNY you will get less and if in eg Moscow Russia you get about 20 minutes/day in mid winter.
Solar power is not recommended in Moscow Russia in mid winter.
To find your city's SSH (sunshine hours) use either Gaisma's serach or usually quicker use Gargoyle and enter just gaisma city_name and most cases it will be first hit.
Processing power should be find. Quickest of trials will show you what to expect. Worst case you can walk and chew gum sequentially rather than both together.
2.4 GHz and 432 MHz equipment can exist with ease at the same location with only the most rudimentary of precautions. You can get them to interfere badly if you try hard to do so.
Effectively what you have discovered (your wireless SD shield) is a method of sending serial data wirelessly, from one Arduino to another, via using xBee shield, wireless shields, etc. What you need to do is to connect to the internet. As you have ruled out ethernet and specified wifi, lets talk wifi shields.
There are a number of wifi shields available for the Arduino that you can use, though they are generally much more pricey than ethernet shields.
Here is one from sparkfun,
And another here.
Take a look at the documentation on the http://arduino.cc website for ways to write code for server connection.
Cheers.
Best Answer
The cheapest option would be to use a very simple transmitter like the one found here.
The problem is you'd still need to read the sensor somehow to be able to send the data through the transmitter but that could be accomplished with a microcontroller that is much cheaper than a full-fledged Arduino. Each sensor would use a wireless transmitter to talk to a base (Arduino-based) with a receiver that would then act as the web server. That might be a bit of a daunting task if you're just starting out.