My system is old (see dxdiag below) but are there any tweaks I could do without upgrading hardware that might enable it to handle larger battles without crashing? The issue is that I tend to crash when fighting more than 1 army which is fairly often when facing Huns or Sassanids. Changing to max performance options and shutting down most background services doesn't really help here.

here is my dxdiag

Operating System: Windows 7 Ultimate 64-bit (6.1, Build 7601) Service Pack 1 (7601.win7sp1_gdr.150113-1808)
Language: English (Regional Setting: English)
System Manufacturer: Dell Inc
System Model: XPS 630i
BIOS: Phoenix - AwardBIOS v6.00PG
Processor: Intel(R) Core(TM)2 Extreme CPU Q6850 @ 3.00GHz (4 CPUs), ~3.0GHz
Memory: 4096MB RAM
Available OS Memory: 4094MB RAM
Page File: 1892MB used, 6292MB available
Windows Dir: C:\Windows
DirectX Version: DirectX 11
DX Setup Parameters: Not found
User DPI Setting: 96 DPI (100 percent)
System DPI Setting: 96 DPI (100 percent)
DWM DPI Scaling: Disabled
DxDiag Version: 6.01.7601.17514 64bit Unicode