well... the games you named support dual core cpus (oblivion and dark messiah do anyhow, not sure about fear)... so your best bet is already to invest in a dual core cpu, at least an X2 3600+... ...OCing is always the best way to go to maximize cost/performance ratio too, so you could take that X2 3600+ up to X2 4600+ speeds, or higher, if you wanted to
for OCing, also, an aftermarket cpu cooler is always nice to have (not essential, but keeps noise levels and temperature down)... creating efficient case ventilation by way of cable and fan placement is something to consider also, otherwise, your cpu will pretty much always be running warm, even with a 3rd party hs+f
as far as which memory brand... not really sure... ...ideally, for sam2, you would want low latency/high bandwidth memory, though (but that can also be expensive)... be sure to invest in at least brand name memory at any rate... you definetly want to avoid budget memory anyhow so your system remains stable (value ram isnt always the best idea, even if it is cheaper)... paired memory modules are also worth considering, so you have dual channel performance added, at the very least 1GB (2x512MB), as thats the optimal minimum for smooth gameplay under winXP x86
you might need to consider a PSU upgrade as well (again, brand name over budget, due to reliability and stability issues, investing at least $60 in a new PSU as a general rule)... because your existing PSU might not be able to handle the new loads being set on it (particularly in regards to amperages on the 12v rails)... but, that will also depend on which hardware you choose to purchase too.