I would recommend starting with 7x10^4 cells/well, though it depends on cardiomyocyte proliferation rate. 

If you still have cell density problems, you could try fast forward transfection or change your transfection reagent. 

I usually seed 2x10^4 cells/cm2, and use SilenceMag for siRNA transfection with good efficiency.

http://www.ozbiosciences.com/transfection-sirna/65-silencemag-transfection-reagent-cell-silencing.html

cell count showed that alive cells were ~6.5x10^5 per ml, so i added ~1.25 ml of cell suspension to one well. but now after 48 hrs the cells are very few. and for transfection ~60-80% are needed. now i changed the media. so for how many days i can wait and change the media to get 60-80% confluence and do transfection?

previously for siRNA transfection, i had used lipo2000. but my pcr showed that transfection didn't occur at all. even in negative control. all groups were similar with no significant difference.

60-80% confluency is given for proliferating cell lines, you will not achieve such rate with your primary cells (or you will just transfect contaminating fibroblasts). Usually, you can perform medium change 18-24h after plating in order to remove dead cells and remaining blood cells, and then change your medium every 2-3 days.

Some transfection reagents, like SilenceMag, are more appropriate to transfect siRNA into primary cells. Their formulations with iron nanobeads increase transfection rate.

@all ....many thanks for your guidance.

can i do transfection twice??  like usually we incubate for 48hrs after transfection, if i transfect it again after 24 hrs then would it affect the results? like it would improve the gene knock down or not?