I ran into issues with 2 4870s back in the day. If you don't have an extra slot between cards, I really would keep away from SLI unless you have Extremely good airflow in your case. A side fan blowing between the cards is almost needed.
With that said, I tend to like stock exhaust coolers for SLI setups since they blow the hot air out the back rather than circulating it. Again, if you have VERY good airflow that shouldn't be a concern.