I had a 1 GB RAM which seemed sufficient for a HTPC. The Windows Experience Index gave my memory a decent score of 3.9. My machine, however, performed poorly. My CPU was always pegged and I thought of upgrading the processor.
I found that Win 64-bit OS has a minimum requirement of 2GB RAM to function properly. I got two 1-GB DIMMs from Newegg: http://www.newegg.com/Product/Product.aspx?Item=N82E16820231047
I also used a 4 GB Sandisk Cruzer flash driver for ReadyBoost.
After the upgrade, my machine is very responsive.
Cost: $64.99 with coupon
Windows Experience Index: 4.5