There is really no easy way for me to say it but your pc is REALLY old.
The only 800MHZ ram modules i could find was the DDR2s so im guessing the parts in your pc are around 8-10 years old.
Anyway id recommend trying and set the graphics settings to something like medium/low in the hope it might stop some of the lagging.
Based on the benchmarks of said parts i will say that there isn't much else that can be done other than that.
My professional opinion would be that you should consider building a new pc if you have the financial capability and knowledge, i gladly wanna help you finding the parts that will best suit what you wanna use your pc for if you need it.
And here is a list of the minimum requirements required to play TU for you to look at: