Help upgrading RAM for my old PC?

I want to upgrade my RAM in my Dell. I'm not sure the model but it is an old model from 2000-2005. I want to know if I should do something before I change the Ram completely. I know i need the same kind in the pc at one time so I can't combine what i have now with the new stuff. I'm not sure if I need a certain kind or model of RAM?

Also, Would I be able upgrade my CPU as well or would I need a new motherboard?

Trying to play Minecraft without lag:/

Why can't I see any pokemon in pixelmon 3.0?

My pixelmon was working perfectly a few minutes ago, but then it ran out of memory so I restarted minecraft and went into the world and I couldn't see any pixelmon. None spawned none were in my party it couldn't spawn any, any ideas on what is going on?

Why are my browsers suddenly so slow? And also games like minecraft?

I'm not sure if the two are related because this happened around the same time. I use chrome and firefox. One day out of nowhere they started getting really slow. Firefox will sometimes freeze the moment it is opened. They are generally just slow at performing tasks that would be near instant before.

Minecraft is also horrendous to play now. I literally can't even turn around in the game without experiencing a lag spike. The worst thing is that if I exit the game, it will crash and then won't close! This is happening every time now! It used to be so smooth.

The only things I have done recently is uninstall advanced system care and switch it to avast. I have since uninstalled avast. I found that it was too interfering, but uninstalling it did not change anything so that probably isn't it. I have also downloaded ccleaner but that is a great piece of software so I kept that.

I just can't think what is causing this. Any ideas? Thanks

(please don't simply advise to delete my browser history. Ccleaner takes care of that)

Any fun in depth mmorpgs?

Lately i have noticed that i have been enjoying the games that make me think and plan (char advancements and how i handle the situation) so much more than i do games like cod, skyrim, minecraft etc. I love dark souls and loved mech warrior online even tho it destroyed my comp but i was wondering if there were any mmorpgs that were as in depth with char development and strategy

Good gaming rig on budget?

I've decided that i want to build to build a gaming rig within a budget of 600-700$. So i went on memory express (memoryexpress.com) to start searching for parts (which is the computer store in Edmonton, Alberta) and i came up with this list. Please let me know if these parts are capable of playing games like minecraft (Heavily modded), TF2 and possibly skyrim. If you have any ideas of what would fit better in this rig that stays within my budget please let me know!
Thanks

List:

Case: Silverstone precision PS08 Micro-ATX Case
Cooling: ARTIC F12 Rev. 2 PWM Fan, 120mm
Memory: RipjawsX Series 8GB PC3-12800 Dual Channel DDR3 (2x4GB)
Motherboard: MSI FM2-A55m-E33 w/ DDR3 2133, 7.1 Audio, Gigabit Lan, PCI-E, CrossfireX
Processor: Intel i3-4330 Processor, 3.50GHz w/ 4MB Cache
Power Supply: Silverstone Strider Essential 700W Power Supply
Hard Drive: 750GB Blue SATA III Notebook Hard Drive w/ 8MB Cache
Graphics Card: Radeon HD 7750 Core Edition 1GB PCI-E w/ DVI, HDMI, DisplayPort

Thanks Again
Be sure to let me know if what I should change
Liam

Added (1). Case: $39.99
Cooling: $6.99
Memory: $89.99
Motherboard: $44.99
Processor: $159.99
Power Supply: $79.99
Hard Drive: $69.99
Graphics Card: $129.99
Mouse: $14.99

Total Price: $636.91