In performance , you will notice no differenceespecially if you play games that need high performance in graphics like car racing, Moto Grand Prix............I changed mine latlely and was very disappointedNow I'm thinking to stop playing games on computer and use a PS3 instead.
An 8500 will net you around 12.8 GB/s, a 9500 will max out at 25.6 with GDDR3. The 9500 will be about twice as fast as your old card so it should make a big difference.Of course a 9800 GTX runs at 70.4 GB/s so it's all relative. Look here for the chart: http://www.hardwaresecrets.com/article/132
Yes, you will notice a large difference. DDR3 memory is much fast than the older DDR2 memory. So, even though it has the same amount of ram, the memory clock will be much faster