There is a bug (969532) with V$RESOURCE_LIMIT prior to 8.1.7 that you could be hitting. The count of transactions is not properly decremented for the termination of distributed transactions, and so the SGA transaction table can appear to be fully used when it is not really. If it were indeed fully used, you would get an ORA-1554 error rather than waiting. Run the following query as SYS to check how full your transaction table has really been.
select count(*) from x$ktcxb where ktcxbflg > 0;
Don't be shy to raise the value of this parameter if you need to. The only
impact is a little more memory usage. However, an unnecessarily high value for
the sessions parameter will slow down queries against certain V$ views. This also affects the performance of the SQL*Plus autotrace facility.
In another database the value of the transactions paramter is 300 and max utilisation is showing 300. Does it mean the system might have waited for resources. Current utilisation is also showing 298, 295, etc. What is the impact of having bigger value for transactions?
Current value of sessions is 435; processes is 400
Have a look at V$RESOURCE_LIMIT. It will tell you what your current (and peak) usage of these resources is (and has been).
In an init.ora file we have sessions = 335 and transactions = 369. I assume sessions is max. sessions on instance and transactions is max. active transactions on instance. Is there a calculation for these parameters? Is it best to take the defaults? A previous DBA had put these into the init.ora file and I wondered why?