Worth the Upgrade?

Upgrading my sister’s system for her, and about to order the CPU (Q6600) and wondering about grabbing another one for $17 for the HTPC in the family room. Also a Dell like hers, checked the CPU support list and should work. I recently switched it from Win7 to OpenElec so starts faster and using fewer resources (I assume). Seems to run pretty good, but a touch laggy when scrolling through the video file lists.

Is it worth the $17 for an upgrade? or, not really?

Dell 745 desktop, 2GB ram, a Dual Core E6600 and a cheap fanless Radeon 5450 1GB video card.

Not sure if extra cores really matter much for video playback.