So, the first question is regarding your form factor, there could be 2 - MT, mini tower and ST, slim tower. Check the pictures below and I identify which one you have
So if you have MT - the Inspiron 545 uses socket 775 and the only quads that it appears to have been configured with MT were the Q8300, Q9300 and Q9400.
Now little comparison
E5300 benchmark is 1077,
Q9400 benchmark is 3455, so this is very good upgrade.
There is only one small caveat - Q9400 is 95 watt and E5300 is 65 watt, I am checking now CPU cooler compatibility.
Found it, ha, it was so easy - Dell uses the same CPU coolers across many Inspiron models, here we go.
E5300, 65 watt (yours) is using C957N/ 0C957N,
cheapest new for $19.84
Q9400, 95 watt (maximum possible, according to credible reports) is using C955,
can be found for only $16.24
Now, I will look into RAM in the next post.
P.S. I seen you posted already, slow down, Inspiron is the easiest to upgrade Dell cheap model, others like Optiplex are much more difficult.