Upgrade my Dell Inspiron laptop for PenTesting

I have a Dell Inspiron N5110 & wanna upgrade it to start practising pentesing with virtual machines. What do u suggest from specs below to upgrade (I have Kali Linux, Tails OS, XP, Metasploitable etc)...

When I use virtualbox with Kali (attacker) & win xp (victim) the laptop becomes terribly slow.

Operating System

Windows 8.1 Enterprise 64-bit

CPU

Intel Core i5 2450M @ 2.50GHz   60 °C
Sandy Bridge 32nm Technology

RAM

4.00GB Single-Channel DDR3 @ 665MHz (9-9-9-24)

Motherboard

Dell Inc. 0FXK2Y (CPU 1)    61 °C

Graphics

Generic PnP Monitor (1366x768@60Hz)
Intel HD Graphics 3000 (Dell)
1023MB NVIDIA GeForce GT 525M (Dell)    55 °C

Storage

465GB Hitachi HTS547550A9E384 ATA Device (SATA) 42 °C

Optical Drives

DTSOFT Virtual CdRom Device
MATSHITA DVD+-RW UJ8C1 ATA Device

Audio

IDT High Definition Audio CODEC