ok, then, real-time data it is. No, it takes up hardly any memory, or rather, the program displaying, tracking and recording the markte data is no more a resource hog than your browser (22MB), spreadsheet (9MB) or word processor (16MB) when idle.
The amount of memory used and cpu load will depend on how many tickers (different stocks) you've viewing at once, the time intervals you've set such as 1 second (continous stream), 5 seconds, 30 seconds, etc. so assume the program you are running will take 100MB idle and 500MB in full swing.
With my spreadsheet running using 9MB, I opened up about 20MB of files. The new memory footprint is now around 90MB. Unless your files are very large I still think 4GB will be plenty.
Also, you're be looking at market news in real time as well in some format. Even more reason for the fast internet connection.
I'll clarify some points. Overall you need speed in the system you're building (not high 3DMark scores) and the capacity to keep trading when something attempts to interrupt it (insurance).
System
Dual cores E8500, E8600*, PII X2 550
PII X4 940, 955, 965*, Q9650
RAM: 4GB DDR2-1066*, or DDR3-1333 if the motherboard you get for the PII X4 is DDR3.
Motherboard: get one with on-board VGA and/or DVI. Just in case your video card dies one day (insurance).
Hardrives: 2 x SSD* drives or 2 x HDD in RAID1. This mirrors the data on each drive so if one drive fails the other keeps on going. Solid State Drives will be quicker but more expensive.
Graphics cards: 2D performance is what you need, not 3D CAD or games. A single card with three outputs is the best choice, so just ask the local computer shops what's current.
Power supply: something like a Gigabyte ODIN GT.
Case needs good ventilation, optical drive can be anything.
* my choices for each.
Insurance
Ask yourself, "I have 100K of open positions using CFCs that are good til cancelled. What happens if...?"
The electricity fails.
The internet connection fails.
A video card dies.
A hard drive goes crazy.
Malicious software gets hold of my computer.
and so on.
$100K of CFCs are quite likely to be leveraged up to 95% covering $2 million underlying stocks. You don't want to be in a situation where you cannot close your position out because of a failure at your end (not the exchange).
UPS are uninterruptable power supplies. The provide electricity if the mains (utility) fails. There are usually three levels of UPS, so go with the thrid type which is double-conversion.
Second internet connection as back up. Medium speed, different carrier or provider than the primary, different type if possible.
Network Attached Storage (NAS) for backing up everything. Usually two hard drives in RAID1, but one drive will do. It's better than nothing. Or off-site back-up.
Good firewall and simple anti-virus measures (Malwarebytes, Spybot S&D) or a fully paid Big Name anti-virus. It's up to you. Depends on how many porn and casino sites you're visiting while trading
Maybe a second computer, but make it a laptop with wireless broadband. Doesn't have to be super quick. It's for the occasion when you need to escape the burning building while in the middle of said $2 million trade. Hopefully you won't need it.
and so on. Plan for these contengencies at your end. Only you can decide the risk involved and either dismiss it or, if accepting it, then implementing a course of action to guard against it.