Dilemma: Add a host or add memory

Hi, we are currently in a dilema. We currently support an ESXi 4.1 environnement that contain 4 host(Clustured, HA, DRS). Here's the setup:

HP DL380 G6

2 X E5540

64 Gb Memory

The cluster actually hosts 140 VM desktop, servers, prod ans lab and so far it runs flawlessly but for HA consideration and futur need for news servers we have two choices. For almost the same price we can either upgrade every hosts to 128 GB of ram or add another host. The reason it is so expensive to add memory, is because every memory slots are full and we need to new ones.

I need opnion of some expert because so far we're not sure what to do.

Thanks for your help