Higher Order Filters
About dual and quad opamp ICs: the performance specs such as offset voltage, open loop gain and gain-bandwidth product are given for one opamp.

About the instrumentation amp gain: You must know how much DC offset and how strong signals can be expected from the sensors in your application. This is the very basic data for succesful design. You must do experiments and measurements for them if you cannot find it from reliable sources. Most of us do not know enough about EMG for good numerical answer. I believe it would be useful to include a preamplifier with differential output into the sensor. But that's only an opinion, I have no numerical facts.

About the filter:

Cascading second order filters to get higher order filtering is a good idea because it gives good control over the effects of component tolerances and make steep filter slopes possible. Cascading first order filters cannot provide steep filtering. That's a mathematical fact. To see it one must be able to understand transfer functions with complex numbers.

Cascading several identical well designed filters isn't the right way to good performance. It can be useful if you cannot perform proper design procedures or the specs are too fuzzy. It's possible to iterate towards a good result by experimenting altough there's no quarantee for it. Simulation makes succesful development by experiments possible in practice.

Total gain of cascaded filter blocks is the product of single block gains assuming the the effect of loading variations can be negleted (in decibels the gains must be summed). If you have passive circuits, the loading varies radically and the resulted gain is very complex to calculate.

Filter design starts from the wanted filtering effect. To be able to specify it you must have good data of expected interferencing noise signals and of the signal which you want to keep clean. The wanted filtering effect spec is numerical: 


passband limit frequency
allowed passband gain variation
stopband limit frequency
wanted minimum stopband attenuation
wanted special frequency attenuations (for ex. you surely want to specify some special treatment for the mains ac frequency)
allowed passband phase nonlinearity


These are the limit specs for the wanted mathematical transfer function. Plenty of good transfer function models (Butterworth, Tsebychev, Bessel, Elliptic) has been developed and you must select which fits to your spec and calculate the parameters for it. This all is very basic stuff which has been well documented in filter synthesis textbooks tens of years ago. You can also find plenty of software to do all calculations. There's software that also help in the next step.

The transfer function must be splitted to second order blocks which can be built using common opamp filter circuits. There are also available special ICs for filters.

Finally you must simulate the design - especially the effect of the component tolerances and noise. Noise can be also measured from a test circuit, but the tolerances must be simulated if you are going to build several units.