i try to keep everthing up to date , as in applications and codecs updated to latest core codec today actually , i dont really run mame games on here just snes n64 dreamcast & neo-geo but i dont run them through mediaportal i may have misunderstood an earlier question i also use tune up utilities as the it was quite sluggish after i built it and after this is went super fast but i think it only benifit older hardware as newer pc got more than enough power TuneUp United Kingdom - TuneUp Utilities 2008 thats the link
I am currently running a very similar system that I built a year or so ago.
Athlon XP 2800+
2x 512Mb DDR1 PC3200 Ram sticks
Western Digital 80 Gb HDD
XP SP2 w/ MediaPortal 1 RC 3
ATi 9250 AGP Card
Hauppauge Nova T 500 DVB-T tuner
Hauppauge DVB-T / Analogue tuner
DVI -> HDMI output.
I runs quite well. I was using 0.2.3.0 for a while but I recently upgraded to 1.0.0.0 RC3, frankly I think that 0.2.3.0 worked better on the hardware. Still, it may just need some more tinkering