Do not dilute your RNA. Generally we use 0.1-1.0 ug of total RNA for reverse transcription using Superscript. Once the cDNA reaction is done, then the cDNA gets diluted 1:10-1:20 and 1ul of this is used for real time qPCR. You should be able to assay for many genes with different primer pairs from one cDNA reaction.

Since your RNA Samples have a concentration of 500ng/uL set up a 20uL c-DNA synthesis for 1 ug of RNA and store remaining RNA at -80 degrees. In this way your c-DNA concentration in 20uL would be 50ng/uL. After that you dilute your 20ul of c-DNA product with 180uL of Nuclease free water making your final concentration 5ng/ul. So from that you could set up 10uL RT PCR reaction with 2uL (10ng) of c-DNA. You can store this c-DNA at -20 for a month or two. After that you could take your RNA that is stored at -80 to synthesize new c-DNA.

Use the RNA starting amount recommended in your kit. You'll use housekeeping genes to standardize the samples AFTER cDNA synthesis.