Up-sampling input signal
why do you want to upsample?  upsampling comes at a cost in the digital domain, and can save expense in the analogue domain.

For example if you have a signal with content that you are interested in the range of 0-3 kHz you can sample using inexpensive a/d at 64 ksps using a one pole passive lower pass anti alias filter.  Once in the digital domain you can pass it through a IIR/FIR multipole low pass and perform decimation (drop seven out of eight samples) so you have 8 KS/s with no aliasing issue and ALL the signal information with no spectral overlap.

On the other hand up sampling a 8 KS/s to 64 KS/s signal will not give you any benefit.  It will reduce the resolution of information you are interested within the 0-3 kHz range, increase memory requirements, and increase processor usage.  Of course you can define smaller frequency bins, but this comes at a computational and memory cost.