Your x800 Pro is much better card than the x1650 XT (except for not having SM3.)
If you look at the
VGA Charts you can compare the x800 XT (which is just a small step ahead of the x800 Pro) and the x1650 Pro (which is equal to the x1650 XT) and you can see that the x800 XT beats the x1650 Pro in every benchmark, usually by a large margin.
Benchmark Data:
Radeon X800 XT R423, 500 MHz, 1000 MHz, 256 MB GDDR3
vs.
Radeon X1650 Pro HM RV535, 587 MHz, 1386 MHz, 256/512 MB DDR3
Note: The word "Card" below refers to the Radeon X800 XT.
Half Life 2: Episode 1 - 1024*768 - No Eye Candy - Card is 80.5% faster
Half Life 2: Episode 1 - 1024*768 - Card is 44.3% faster
Half Life 2: Episode 1 - 1280*1024 - Card is 45.6% faster
Half Life 2: Episode 1 - 1600*1200 - Card is 52.7% faster
Hard Truck Apocalypse - 1024*768 - No Eye Candy - Card is -0.4% slower
Hard Truck Apocalypse - 1024*768 - Card is 41.6% faster
Hard Truck Apocalypse - 1280*960 - Card is 51.6% faster
Hard Truck Apocalypse - 1600*1200 - Card is 78.4% faster
Oblivion: The Elder Scrolls 4 (Outside) - 1024*768 - No Eye Candy - Card is 116.5% faster
Oblivion: The Elder Scrolls 4 (Outside) - 1024*768 - Card is 153.3% faster
Oblivion: The Elder Scrolls 4 (Outside) - 1280*1024 - Card is 160.0% faster
Oblivion: The Elder Scrolls 4 (Outside) - 1600*1200 - Card is 175.0% faster
Oblivion: The Elder Scrolls 4 (Inside) - 1024*768 - No Eye Candy - Card is 98.2% faster
Oblivion: The Elder Scrolls 4 (Inside) - 1024*768 - Card is 114.8% faster
Oblivion: The Elder Scrolls 4 (Inside) - 1280*1024 - Card is 113.1% faster
Oblivion: The Elder Scrolls 4 (Inside) - 1600*1200 - Card is 103.6% faster
Prey - 1024*768 - No Eye Candy - Card is 28.7% faster
Prey - 1024*768 - Card is 26.3% faster
Prey - 1280*1024 - Card is 24.0% faster
Prey - 1600*1200 - Card is 45.6% faster
Rise of Nations: Rise of Legends - 1024*768 - No Eye Candy - Card is 20.2% faster
Rise of Nations: Rise of Legends - 1024*768 - Card is 22.7% faster
Rise of Nations: Rise of Legends - 1280*1024 - Card is 7.7% faster
Rise of Nations: Rise of Legends - 1600*1200 - Card is 21.2% faster
Titan Quest - 1024*768 - No Eye Candy - Card is 29.7% faster
Titan Quest - 1024*768 - Card is 41.0% faster
Titan Quest - 1280*1024 - Card is 24.0% faster
Titan Quest - 1600*1200 - Card is 2.7% faster
3DMark06 (v1.0.2) SM2.0 - 1024*768 - No Eye Candy - Card is 14.1% faster
3DMark06 (v1.0.2) SM2.0 - 1024*768 - Card is 16.8% faster
3DMark06 (v1.0.2) SM2.0 - 1280*1024 - Card is 17.1% faster
3DMark06 (v1.0.2) SM2.0 - 1600*1200 - Card is 18.4% faster
Card is 54.4% faster on average (excluding 3DMark06 benchmark).
In addition to that data, if you are interested in the x1950 Pro, it is on average 40% faster than the x800 XT. If you want the performance numbers for it, just ask.