welcome to the forum.

first amd athlon 64 x2 is better than p4 d "dual core netbrust core".less power requirement ,less heat , and better performance.

so now for completely new pc the new intel conroe architecture is the way to go .conroe core cpu is about 20% faster clock for clock than amd x2 and about 90% faster than older p4 d.

 u can get the lower-end conroe cpu for about 183-190$.

about heat of amd cpu : this completly wrong maybe u are talking about old athlon xp day.but since A64 amd run cool and have good performance.

to go with intel conroe u must get at least the 965 chipset on good exmple is the gigabyte GA-965P (200$ range) witch offer good capability to overclock conroe cpu 400 fsb+.u should get into the 3.3-3.6ghz range easly this sould corespond to about 4 ghz amd cpu(as a refrence the highest amd fx 62 is clocked @ 2800ghz).or to about 7 ghz from old intel core.with i think is more than enough.

if u look to safe money and go with lower cpu i can go with athlon 64 x2 3800+ about 150$ and am2 motherboard about 100-120$ "non sli".

x2 4400+ vs x2 4200 vs x2 3800+ the only defrence is 512 in cache and/or 200 mhz in frequency .the lowest one the 3800+ can clock without problem the the speed  of 4800+ and higher"i am talking about overclocking".

the cache did not play big role with amd 64 .the cpu  is more depondant from the working frequency.

for the memory 2*512 still pretty enough to run current game but some game such battlefiled 2 require over 1 gig to run with all detail to max.all today cpu has dual channel memory so using 2 slot u can double your memory brandwith.

2*512 is better because with 2*512 u get the double memory bandwidh for expm with single memory slot u get 1*3200mb/s (assuming that the memory is ddr 400)with 2 slot u get 2*3200mb/s =6400mb/s(the same as abrove) but u doubled cpu memory performance.

for hdd i would go with largest model where there is the best cost per 1 gig  . 320 gig is good deal for the moment

-seagte vs hitachi : seagate have 5 years wrantly .hitachi is a bit faster u are ok with borth.

graphic card: if u have 100$ budget the card 7300 gt is the best that u can get remenber to go with inno 3d or palit they borth have version with ddr 3 memory the card come with 500GPU/1500memory.those card can clock high and can reach the performance of 7600gt.

for the monitor what is the price range?

feel free to ask more question .

you are welcome:-)